Yeah, you can't enable them both at the same time. How crossfire works is that they pretty much take turns rendering the frames, splits the load between each one. You'll see more improvement on high res/low framerate games. Not so much on older ones.
The 5450 is a damn weak card. No way to really argue with that, cuz there was better stuff all the way back in 2002, measuring with just bandwidth. It doesn't even score 1000 in 3D mark vantage. Don't think that you'd see much of a boost anyway.
If I were you, I'd leave the card in an old computer and save up for another 5770.