Hi !
(Not a technical answer but from my user experience)
As far as I know, it seems not to be a problem to use two different GPUs.
Indeed, it's actually the CPU which send calculations to be done by a GPU in some step of the workflow.
So if both of your GPUs are working in PhotoScan, it should not a problem to use both at the same time.
Regards
P.S. About power usage of both GPUs, I guess it shouldn't be different from the individual power usage of each one (as they work in parallel). The only limit I may see would be about the ability of the CPU to distribute the work to both GPUs and calculate at the same time.