"And I will grant authority to my two witnesses, and they will prophesy for twelve hundred and sixty days, clothed in sackcloth" (Revelation 11:3).
Usually when one deals with the two witnesses of Revelation 11:3-13, the discussion centers around the identity of the pair. Even though I believe they will be Moses and Elijah, that will not be my focus. The emphasis of this article is whether the two witnesses will minister in the first or second half of the seven-year tribulation. I think they will appear in the first half of the tribulation, meaning their death and resurrection will take place at the mid-point rather than the end of the tribulation.
Most literal interpreters believe that the second half of the seven-year tribulation will be the time in which the Antichrist or the Beast dominates the world and issues the mark of the Beast. This is why the Jews will leave Jerusalem and flee to the mountains at the mid-point of the tribulation, because the second half is the great tribulation dominated by the Beast. However, the first half of the tribulation will be the time in which the two witnesses and the 144,000 Jewish witnesses perform their ministries.
Why the First Half?
Literal interpreters are split evenly between those who see the two witnesses in the first  or second  halves of the seventieth week of Daniel. John Whitcomb has made the following arguments in favor of the two witnesses appearing in the first half of the tribulation:
First, there seems to be an intentional distinction between the time of the Gentile occupation of the Temple's outer court and the city, and the time of the two witnesses, by means of the different time-units used: 42 months for the Gentile domination and 1,260 days for the two witnesses. If the same time period is intended for both groups, why is not the 42-month time-block sufficient to cover both? 
"It seems possible that the 'two witnesses' of Revelation 11:3-6, who have irresistible authority in Jerusalem during the first three-and-one-half years," notes Whitcomb in his commentary on Daniel, "will also be instrumental in arranging the terms of this covenant with the 'little horn,' for not until they are killed by him (after he 'comes up out of the abyss') is he able to break the covenant and terminate the sacrificial system."  This means that the two witnesses at the beginning of the tribulation could oversee and then protect the rebuilt Temple and reestablished the Jewish worship system since they are given supernatural powers.
"And if anyone desires to harm them, fire proceeds out of their mouth and devours their enemies; and if anyone would desire to harm them, in this manner he must be killed" (Rev. 11:5).
This is why the Beast cannot go into the Tribulation Temple until the two witnesses are out of the way. This will occur at the midpoint of the tribulation.
"And when they have finished their testimony, the beast that comes up out of the abyss will make war with them, and overcome them and kill them" (Rev. 11:7).
Notice the emphasis upon the fact that they will only be allowed to be killed by the Beast when our Sovereign Lord says, "when they have finished their testimony." God is in control. This allows the Beast to defile the Temple in the middle of the seventieth week (Dan. 9:27; Matt. 24:15–21), thus supporting the notion that the two witnesses minister in the first half of the tribulation.
Whitcomb further says, "the Lord Jesus issued this command to Jews of the tribulation period: 'when you see the "abomination of desolation," spoken of by Daniel the prophet, standing in the holy place...then let those who are in Judea flee to the mountains...For then there will be great tribulation such as has not been since the beginning of the world until this time, no, nor ever shall be' (Matt. 24:15-21). Here an obvious question arises: Would the two Jewish witnesses remain in Jerusalem during the 42 months of Antichrist's dominion if the Lord Jesus, their Messiah, told them to flee to the mountains?" 
Next, Whitcomb points out that, "if the 1,260 days occur during the last half of the Week, then the entire world would be celebrating the death of the two witnesses for three-and-a-half days after the Battle of Armageddon and the destruction of the Antichrist! This is very difficult to imagine."  It is not only difficult, it is impossible to imagine that a celebration would be going on as Jesus judges the world at His second coming. Gary Cohen explains:
At the end of the second 3½ year period the Beast's followers are lamenting over Babylon's doom and are gathering for the great battle at Armageddon, and finally slain by Christ, whose coming is surrounded with the powers of the heavens being shaken (Rev 16-18; 19:11–21; Matt 24:29–30). This picture does not harmonize well with the 3½ days of rejoicing and gift giving in which the earth dwellers participate following the murder of the witnesses (Rev 8:10). This discordance between the end of the second 3½ year period and the 3½ days, following the end of the 3½ year ministry of the witnesses, makes it most unlikely that the prophesying of God's two servants take place during the latter half of the week. 
Additionally, Whitcomb declares:
Putting the two witnesses into the last half of the Week compromises the totality of Antichrist's dominion during that same period. How can he bring fire from heaven upon his enemies (through the False Prophet, Rev. 13:13) if the two witnesses are simultaneously bringing fire from heaven upon their enemies (Rev. 11:5)? We are clearly dealing with two different time periods: the first half of the Week with the overwhelming power of the two witnesses, and the last half of the Week with the overwhelming power of the Beast and the False Prophet. When the world asks the rhetorical question, "Who is able to make war with [the Beast]?" (Rev. 13:4), it seems obvious that no one can answer, "The two witnesses are able to make war with him," for their 1,260 days of ministry will have ended, and they will be gone. 
Finally, Whitcomb notes: "Our Lord stated that 'Elijah is coming first and will restore all things (italics added)' (Matt. 17:11). Whoever 'Elijah' turns out to be, his spectacular success (under God) in bringing Israel back to her Messiah must be during the first half of the seventieth week, for Isaiah prophesied that Israel will have given birth 'to her children' as soon as her time of tribulation begins (Isa. 66:8)." 
I believe that a very strong case can be made for the two witnesses exercising their ministry in the first half of the tribulation; not the second half. As noted in the points above, it makes sense that the positive points for this position can easily fit into the first half, while many problems arise when one places them into the last half. It also appears to me that the 144,000 Jewish witnesses of Revelation 7 and 14 belong in the first half as well. This means that the first half of the tribulation is likely the primary time in which hundreds of millions of Gentiles are saved, as well as the majority of the Jewish Remnant. The focus of the second half of the tribulation will be on those who are saved in the first half making it through the second half in order to greet Christ at His return. Maranatha!
 Some who see the two witnesses in the first half of the tribulation include: J. Allen, What the Bible Teaches: Revelation (Kilmarnock, Scotland: John Ritchie, LTD, 1997), p. 285–86; Gary G. Cohen, Understanding Revelation (Chicago: Moody Press, 1978), pp. 251–54; Arnold Fruchtenbaum, The Footsteps of the Messiah (Tustin, CA: Ariel Ministries, 2003), pp. 152–53; Tony Garland, A Testimony of Jesus Christ—Volume 1: A Commentary on the Book of Revelation (Comano Island, WA: SpiritAndTruth.org, 2004), p. 448; Robert Govett, Govett on Revelation, 2 vols. (Hayesville, NC: Schoettle Publishing, 1981 ), vol. I, p. 513; David Jeremiah, Escape the Coming Night: Messages from the Book of Revelation, 4 vols. (San Diego: Turning Point Ministries, 1994), vol. 2, p. 121; Tim LaHaye, Revelation Illustrated & Made Plain (Grand Rapids: Zondervan, 1975), p. 152; David Larsen, Jews, Gentiles and the Church (Grand Rapids: Discovery House, 1995), pp. 274, 293; Hal Lindsey, There's A New World Coming: An In-Depth Analysis of the Book of Revelation (Eugene, OR: Harvest House, 1984), p. 152; Henry M. Morris, The Revelation Record: A Scientific and Devotional Commentary on the Prophetic Book of the End Times (Wheaton, IL: Tyndale House Publishers, 1983), p. 198; J. Dwight Pentecost, Things to Come: A Study in Biblical Eschatology (Grand Rapids: Zondervan, 1958), p. 309; J. B. Smith, A Revelation of Jesus Christ (Scottdale, PA: Herald Press, 1961), p. 171; Gerald Stanton, Kept from the Hour (Miami Springs, FL: Schoettle Publishing Company, 1992), pp. 187–88; Charles R. Swindoll, Swindoll's New Testament Insights: Insights on Revelation (Grand Rapids: Zondervan, 2011), p. 158.
 Some who see the two witnesses in the second half of the tribulation include: Arno C. Gaebelein, The Revelation (Chicago: Van Kampen Press, n.d.), p. 69–70; Edward Hindson, The Book of Revelation: Unlocking the Future (Chattanooga, TN: AMG Publishers, 2002), p. 122; Mark Hitchcock, The End: A Complete Overview of Bible Prophecy and the End of Days (Carol Stream, IL: Tyndale House Publishers, 2012), pp. 351–52; Jack MacArthur, Expositional Commentary on Revelation (Eugene, OR: Certain Sound Publishers, 1973), p. 233; John MacArthur, The MacArthur New Testament Commentary: Revelation 1–11 (Chicago: Moody Press, 1999), p. 298; Randall Price, The Temple and Bible Prophecy: A Definitive Look at Its Past, Present, and Future (Eugene, OR: Harvest House, 2005), p. 320; Robert L. Thomas, Revelation 8-22: An Exegetical Commentary (Chicago: Moody Press, 1995), p. 89; John F. Walvoord, The Revelation of Jesus Christ (Chicago: Moody Press, 1966), p. 178.
 John C. Whitcomb, "The Two Witnesses in Revelation 11," Paper presented at the Pre-Trib Study Group Conference, December 2008, p. 2; http://www.pre-trib.org/data/pdf/Whitcomb-TheTwoWitnessesFirst.pdf.
 John C. Whitcomb, Everyman's Bible Commentary: Daniel (Winona Lake, IN: BMH Books, 1985), p. 134.
 Whitcomb, "The Two Witnesses," p. 3.
 Whitcomb, "The Two Witnesses," p. 3.
 Cohen, Understanding Revelation, p. 134.
 Whitcomb, "The Two Witnesses," p. 4.
 Whitcomb, "The Two Witnesses," p. 5.