The Father is called the Father 4 times in the book of Revelation, so Jesus is not the only one mentioned. The first time 'Father' is used is in verse 1:6 which says "... priests to serve his God and Father...". So the Father is Christ's God.
But God is already mentioned earlier. In verse 1 it says "The revelation of Jesus Christ, which God gave him...". That verse distinguishes between Christ and God. Then verse 2 says "...the word of God and the testimony of Jesus Christ". So that verse not only distinguishes between Jesus and God, it also makes it clear that Revelation has not only the testimony of Jesus but also the word of God. So verse 8 does not have to be the testimony of Jesus but simply the word of God himself. Verses 4-5 say "...from him who is and who was, and who is to come, and from the seven spirits before his throne, and from Jesus Christ, who is the faithful witness...". Notice that passage distinguishes between the one who is and who was and is to come, and Jesus, so it's two beings, the first one being God, the second one Jesus. And about "who was and who is to come", that sounds a lot like Alpha and Omega. So in verse 8 the Alpha and Omega can be the Father rather than Jesus.