This verse actually reads, "I will give to these witnesses of Me." He does not say that He merely possesses them, that is, that they are His witnesses. Instead, He says that they witness "of Me." They point everybody in the world to Jesus Christ and thus on to God the Father. It is their job to witness of Him.
The whole Old Testament points to Jesus Christ, and the New Testament tells His story. So the entire Bible is also a witness of Jesus Christ and therefore of God the Father. In a sense, everything comes down to witnessing of Jesus Christ. What are we called? "Christians." Our whole lives should be totally focused on showing or manifesting Jesus Christ in us. These Two Witnesses are pinnacles of that among men. They will witness of God for 3½ years, in the face of the entire world.
It is interesting how these Two Witnesses correlate with Jesus Christ Himself. We could say that, individually, they will be images or representations of Jesus Christ. God has called us all to be transformed into the image of Jesus Christ, and these Two Witnesses—these two prophets—will show the world in themselves what this means. Their witness will be so true, it will be as if they are two "Christs" walking the earth. Perhaps this is exaggerating things a bit, but it is indeed one of the ways in which a person witnesses, which is why these two prophets are so important.
Richard T. Ritenbaugh
The Two Witnesses (Part One)