Use of 2 video cards

What you're referring to is SLi (NVIDIA) or Crossfire (AMD/ATi)
They can give you more performance than a single card. For example, 2 X GTS450s gives performance on par with a GTX 470.
But you'll need a SLi or XFire compatible motherboard to SLi or CF (Cross Fire). The card models have to be the exact same one too.

Other than that, you can install another card to use as a physx card. This can only be done in the case of NVIDIA cards though. Only NVIDIA cards can be used as physx cards. AMD/ATi can't be used.
For example,
GTX 570 - the discrete card used for graphics and such and
GTX 460 - used as the physx card. :)
Hope this helped.