If thats the case, then you probably wont see any performance benefits of using multiple video cards.
Even if you have a game running in one monitor, and you're browsing the web in the other, you're not going to see any significant FPS loss in the game. Rendering 2D surfaces is trivial to a full speed GPU (meaning one that isn't down clocked due to no 3D apps running).
That said, it is possible. In fact, this is how multiple monitors were used originally, video cards never used to have multiple monitor outputs - you needed a card for each one.