Ah now I understand. Thanks for clearing that up.
I guess there are four possible solutions:
1) Drummer who sings-- good for live shows, but hopefully he doesn't do so in the studio
2) Petrucci sings-- very doubtful; bad vocal abilities, and we wouldn't want it to detract from his playing
3) Rudess patches the vocal parts of MP for live shows-- Also doubtful: DT shouldn't play Portnoy's voice during a live show in this post-MP era
4) Labrie fills in-- my favorite option; I can see him doing trade-offs with himself during live shows. It wouldn't be too bad
EDIT: or what Adami just said. But, we should be aware that that would preclude DT from playing Glass Prison