Using a GTX with an NVS in not unstable, it's a hassle. So much of a hassle that we don't recommend it. Performance is so-so. Eight of your 1080p streams have to travel down the PCIe bus of one card and back up to the other. I'm sure someone here can do the math on PCIe bandwidth, it's safe to say it's far from ideal.
Two GTXs together works better, in so far that you don't have to jump through crazy hoops to get the setup to run. You do run into the same problem that half of your output has to travel via the PCIe buses, which takes power away from performance.
I'm going to give the same advice as Oaktown, the most reliable and optimal solution is to use Datapath graphic extenders. That way all processing is handled on a single GPU, and no extra passing back and forth of textures is required.
I know Datapaths are out of your budget. Keep in mind that anything over 4 outputs required NASA grade hardware not too long ago. Cheaper hardware and software made many things easier and accessible, but still running an 8 or more output setup is a considerable undertaking. If you're able to set it up, you have a unique and highly valuable skill, that not many others have. If there is no budget on the project to do it correctly hardware wise, chances are your own compensation is not valued at what it should be either. In that case, it might be a good idea to reconsider investing the time and effort involved in doing it on the cheap.
Currently the only way to do more than 6 outputs on a single computer reliably without extra hardware, is to use a media server solution like AI or Touch Designer running on Quadro hardware. At which point you're probably looking at a similar or even much higher price tag as two Datapaths and Resolume.