as far as I've seen, there is one thing you need to look out for in compatibility: you need 2 of the same Graphics cards. if you get them at once, no problem, except you'd need really deep pockets to do so.
dual GPU's is different to dual CPU's. dual GPU's means they are guaranteed to split the load equally. applications often only suport one CPU, and that's why often that does not give an advantage.
dual GPU's doesn't have to have support from the game: to the game, there is only one graphics card. but the OS, it knows there are 2.
DirectX and the device drivers are what makes games utilize graphics cards. a program sends instructions to the OS and/or DirectX, which utilize the drivers to work the graphics card(s)
if there are 2 GPU's, the program doesn't see that.