i think you're mistaking something. sure you can run 2 video cards separately, and yes, you can plug them both into the same monitor. But the main question is why? you'd have to switch between inputs to see what the other card is working on. if you want to tie the two together so that they perform better, that's called SLi, and you'd need a 2nd 650 to do it.
The best use for your old 650 would be to use it as a dedicated physx card.