The game as it is, is supposedly locked at 30fps... Hadding a second gpu as a PhysX card wouldn't change that IMHO. It may resolve the floating batmobile or glidding Batman bug but not much else. Hell, I've even seen some video showing the same inside out faces bug that was present in AC also present in this game and they're not even using the same engine.
That issue has an easy fix. You just change a value in the config file. There are much bigger problems that need to be dealt with.