The best way to do this would be to use an adapter to connect your vga to the 7950. If you connect your screen to the 730, it will not be able to take advantage of the 7950's acceleration. Since the 7950 can easily handle 3+ screens, there is little reason for a second card.
If you want to use the 730 to drive an extra screen, you probably can. Sometime AMD and Nvidia drivers don't work well at the same time, but the more recent drivers seem to work better.