Oh I already have the 6450 as an old card. I was seeing if it could give me a small fps boost until I can afford a 7750.
Ah, i'm pretty sure it's possible, so you should see minor increases in something like FPS. But with crossfire there are lots of microstutter and dropped frame issues (with the current drivers at least) that might result in a net loss of practical performance. Your call, it should work so it wouldn't hurt anything to try it.