HEXiT :
that might be an issue... i think nvidia brock there drivers if they find an amd card in there system... they may have changed that policy but you couldnt do it for years. so dont be surprised if 1 or both cards suddenly dont show up as driver enabled.
...well both cards are Nvidia the older one a GTX 460 (should have maybe prefaced that as yes, there is also an Radeon 460). The GTX 460 has only 1 GB of VRAM so pretty useless for rendering. Also I am using Iray which is CUDA based.
I used to work with Lux (Reality plugin) but they dumped development of their hybrid GPU/CPU mode and went to straight GPU or straight CPU mode rendering. Had dual HD 7950s (3 GB) I was planning on using, but by themselves, not enough VRAM to contain a full scene (and unlike Iray, if VRAM was exceeded in Lux, the process crashed rather than dumping to the CPU). I ended up uninstalling the engine, plugin (which was also pretty buggy in the Ver. 4 release) and the two cards as the cards were power hogs (295 W ea at peak load) and straight CPU mode in Lux is measured in geologic time.
The GTX 760 Ti and GTX 460 together would draw less than a single HD 7950 as the 460 would only be running the displays and thus not be driven hard.
Even 4 GB is pretty much the bare minimum, but I have to make do until (if ever) prices come back down to earth for the GTX 1070/1080.