Can I mine with CGMiner or BfgMiner with both an AMD and NVidia GPU on the same MOBO? I cannot find any good articles or webpages on this, is it possible to run CGMiner with both an AMD and a NVidia graphics card installed? I think I can remember reading something way back that said you shouldn't but can. Does anyone know any concrete information on it?
Yes, it´s possible.

But, there are some things that you shound know:

1) Minning Bitcoin with GPU is almost dead, you should have read about other crypto coins. Specifically "scrypt" based ones. 

2) Nvidia are not power efficient for mining... would be better if you just sell the nvidia and buy a amd with the same price. Unless you want to use your nvidia for gamming, video editing... dunno... just not for mining. 

3) CGMiner = AMD ; CUDAminer = Nvidia ... diferentes programs. don´t no if they work well together... if it does, them you are good to go :-) 

hope i had help you.