Vote: Separate drives = Better.
If you install on separate drives it is easier and you have more options after.
You can move either drive to a new machine if desired in the future and you can disconnect either drive at any time, which does sometimes become necessary.
I have one GTX-240 now and it can drive my one monitor fine but it stutters when I plug in the second.
Can I just throw a second card in or do I have to insert some kexts?
Monitor one is 1920x1200(DVI) monitor two is 1920x1080(HDMI) - Just too much for one little graphics card I guess.