Unfortunately, in some games this does occur, though you made it sound like it's happening in all of yours... Not all games are designed to support SLI or Crossfire, and in the occasional game, enabling them actually hurts performance.
Maybe more specifics on the games you're playing and experiencing this issue with, the resolution(s) being used, as well as your full system specs might allow someone to help you further. Unfortunately, I cannot, as I have rather limited first-hand experience with SLI.
I'm currently playing the game "The Last Remnant". I have tried all the resolutions available and it slows down after a few minutes then gets back to speed. I have a T9300 CPU, 4Gb RAM & Windows Vista 32-Bit.
Well, with that game in particular, all I can find is that Square/ENIX doesn't support "mobile" versions of GPUs in the first place. But, most companies say that despite the fact that their titles run just fine on them to avoid the hassle of having to test laptop/notebook compatibility.
What I can't find at all is any reference to SLI systems within their support pages. Perhaps it would be best you contacted them directly for assistance in the matter. They might brush you off due to the lack of direct support for your mobile graphics card version, but they might be able to tell you whether or not the game supports SLI or not.