Hi, folks.
For what it's worth, I've been running multiple GPUs and Photoscan Pro on a Dell Precision 7910.
Inside the case I have a Titan X (12 GB RAM is needed for some large point clouds) and a Quadro K2200 (I typically do not use it for photo-crunching, but there are occasionally, other 3D applications). Additionally, I hae a Netstor NA250 GPU enclosure (through one of the PCIe Gen3 slots) that runs three GTX 980Tis. The enclosure has its own power supply and cooling. Also, each GPU on the motherboard is linked to a separate CPU. The system has approximately 200GB RAM.
When I run a big job (ultra-high with a few thousand photos), the GPU temps rise up to approximately 70-75C during depth map filtering -- that's not too hot, but if I am concerned, I can easily remove the side panel of each enclosure. My GPU cards are not significantly overclocked -- I don't see the point in pushing that as GPUs are only used in on aspect of the processing.
In summary, multiple GPUs can work, and the 980Tis are very good. The main reason I dropped extra money for the Titan X was the GPU RAM for displaying very large point clouds (a few billion points including colors) -- not for photo-processing.
Paco