Question: If the man Antichrist and the Beast Out of Bottomless Pit are one, how is it that the Antichrist reigns 7 years and the Beast Out of the Abyss reigns 3 and 1/2 years?
Response: As to the apparent discrepancy between the seven years of antichrist's rule and the 3.5 years, the first thing to note is that while the Tribulation lasts for seven years, the Great Tribulation is the second half of that period, 3.5 years, etc. It will be helpful here to insert a passage from part 3A of the Coming Tribulation series which collects the pertinent passages:
The Forty-Two Months (of Rev.11:1-2: This period of time wherein the gentiles will "trample" Jerusalem is, as explained above, a reference to the Great Tribulation (which commences with the seventh trumpet directly after the termination of the two-witness ministry discussed in chapter eleven). Our Lord makes a similar reference to the trampling of Jerusalem by gentiles in Luke 21:24, and tells us that this situation of gentile intrusion and conquest will continue "until the gentiles' times have been fulfilled", that is, until His Second Advent brings antichrist's control of Israel and Jerusalem to a violent and immediate conclusion. It will be useful at this point to summarize the various scriptural designations for the forty-two month time period covered by the Great Tribulation:
In Daniel 7:25, the saints of the Most High (i.e., believers) are said to be handed over into the power of the little horn (i.e., antichrist and the Great Persecution) for "a time, times, and half a time", a biblical way of expressing the three and one half years of the Great Tribulation.
In Daniel 9:26, "the people of the prince which is to come" (i.e., antichrist as the ruler of revived Rome) will make a treaty during the last "seven" and break it in the middle of the "seven", that is, during middle of the seven years at the outset of the Great Tribulation.
In Daniel 12:7, the angel speaking with Daniel declares that it will be "a time, times, and half a time" before the persecutions stop and everything comes to an end, that is, the Great Tribulation will last three and one half years.
In Revelation 11:2, the gentiles (i.e., the army of antichrist) will afflict Jerusalem for 42 months, that is, during the entire three and a half year period of the Great Tribulation (albeit under varying circumstances).
In Revelation 12:6, the woman Israel is said to be protected for 1,260 days, that is, during the whole 42 months of the Great Tribulation (expressed in standard 30 day months).
In Revelation 12:14, the woman Israel is said to be protected for a time, times, and half a time, that is, during this same period of the Great Tribulation's three and a half years.
In Revelation 13:5, the unbridled reign of antichrist is said to last for 42 months, that is, for the duration of the Great Tribulation.
The last passage here is the one at the root of your question. The Greek of Revelation 13:5b (and the text printed in many Greek versions is not entirely correct: Sinaiticus [Aleph] has the correct reading) states "it was given to him (i.e., the beast) to do what he wishes for forty two months". The precise language here is in complete agreement with what we find in Daniel 11:36 "then the king (i.e., antichrist) will do as he pleases" (with the Hebrew phrase cirtsono, כרצונו, being an exact equivalent of the Greek ho thelei, o( qe/lei). Clearly, in Daniel chapter 11 antichrist has already completed two campaigns against the southern alliance by the time we reach this verse, so that he has not only been "reigning" for some time, but has also engaged in major military aggression before the period of being able to fully "work his will" begins. And that is the essence of the 3.5 years of the Great Tribulation - there is no longer any restraint upon antichrist, all other major opposition in the world having been dealt with in one form or another. Among the many horrors which follow is the Great Persecution which will yield more martyrs for Jesus Christ than all persecutions since slaying of Abel put together (cf. Rev.7:9-14).
Antichrist's gaining of control over Babylon, his seizure of revived Rome, and his successful conquest of the southern Egyptian led alliance all happen within the first half of the seven years, leaving him with a free hand to persecute the people of God during the last three and a half years - and that is why this 42 month period is called "The Great Tribulation" (cf. Dan.12:1; Matt.24:21).
In God's great grace and mercy, may we be spared from that day, but if it be our lot to suffer through it, may we be blessed to do so in a way that brings honor and glory to our dear Savior who suffered so much for us.
The Coming Tribulation series (a seven part series covering the entire book of Revelation along with all other prophetic passages touching on the end times), deals with all these issues and many more. Here are some further links which address various aspects of those difficult times to come:
Antichrist and his Kingdom (CT part 3B)
Antichrist: Alive and Well and Living on Planet Earth?
Antichrist's 'desire of women' in Daniel 11:37 et al.
Eschatology and the Old Testament
Last Things and Last Judgments
More on Antichrist and his Kingdom
Antichrist and Babylon
Numbers, Letters, and the Mark of the Beast.
The Chronology of the End.
Aliens, antichrist, and eschatology.
The Beast: Some Questions about Antichrist.
Things to Come: The Half Hour, the 144,00, the Book of Life, Rewards, the Beast's Kingdom, the Great Apostasy, and the so-called 'Partial Rapture'.
Things to Come II: Genesis Rapture, Daniel's Weeks, Seven Kings, Signs of the Apocalypse, Tribulational Suffering, Seven Seals, the Bride of Christ, and Mystery Babylon.
Eschatology Issues: The Fig Tree, Ezekiel 38, Joel 3, and the Trumpets.
Eschatology Issues II: Angelic bodies, heaven and hell, Satan and the Nephilim, etc.