Like Revelation 11:4-5, verse 6 has obvious references to the Old Testament. The first miracle—no rain falling in the days of their prophecy—refers specifically to Elijah's 3½ year drought (I Kings 17-18). The second—waters turning to blood—is an obvious reference to the first plague that Moses brought upon Egypt (Exodus 7:14-25). This seems to be a return to the way God's servants worked before Jesus Christ came—specifically focusing on Elijah's and Moses' works.
The word "power" appears three times in the description of the Two Witnesses, once in Revelation 11:3 and twice in verse 6. When somebody says, "These have power," we think in terms of energy or force or strength to do something. However, the implication of "power" here is authority. God gives them the authority, or the right, to cause these things to happen. In a way, they are given carte blanche to do what needs to be done.
In studying the lives of Elijah, Moses, and others of the prophets, we do not often see them going to God and saying, "Now, what should I do at this point? God, you know our enemies are coming, and I'm not sure what I should do." No, they just do whatever needs to be done. In II Kings 1, when the groups of fifty men and their captains come upon Elijah, the prophet was not sitting there and praying at the top of a hill, saying, "Oh, they're getting close. God, tell me what to do." He just called for fire from heaven and destroyed them. So, the Two Witnesses are given much the same authority at the time of the end. These two will have been trained and prepared by God to such an extent that they will know what to do. They will call upon God, and He will answer with power.
We do not find Jesus, for that matter, beseeching God for instruction about what to do. If someone came to Him for healing, He healed him. If someone needed a demon cast out, He cast out the demon. Once one has God's Spirit—and is in line with God's will—then these decisions are easier to make because, as Paul says in I Corinthians 2:16, "We have the mind of Christ." As we grow, we develop more of that mind of Christ, and we should be able to make decisions as Christ would make them.
So these Two Witnesses will be very much like Christ. They are witnesses of Him, as it says in the literal translation of verse 3. They are, in a way, some of the best representatives of Jesus Christ and His character that will have ever walked this earth. They will act like Christ as much as any two men can, and people in the world will see these two people as like Christ—and eventually treat them as they treated Christ.