I'm just going to answer my question myself. As it turns out, the nvidia cards have some sort of problem with the motherboard; so, when there are two nvidia cards (I'm assuming this pertains specifically to identical cards) a bug in Aperture's render algorithm doesn't allow for proper rendering, regardless of how many monitors are connected to one card.
The recent 2.1.1 update fixes this issue. After installing, I relaunched the application and tested editing a few photos. The problem appeared to have been solved. To make sure, I rebooted the computer and launched and relaunched the application, testing random photos every time, and the bug didn't reappear.
So, it seems that the update is the answer.
However, this may not be the case for every user. Switching a different card will be your only option at that point. So, if you have two nvidia 7300s installed, replacing one with, say, a nvidia 8800 will do the trick.
