here's what you do. When you get through Tails's part, take Sonic back up with the purple gem and go through Tail's part all over again, as Sonic. In the last Tails room, you will not only find Tails standing there, but Sonic as well!
When I found this, the 2nd Sonic was dead and had yellow shield on. :D
Looks like Sonic got to meet his past self after all! :D :D
(now time to count how many people already knew about this)
edit: OH SNAP if you touch Tails this way, you play as him again! The door is locked though so you have to fly all the way back. There's no point to that though, since that's the end of the act. aww :P