Thank you very much for a reply, Niko! I have just carried out some test on a single still frame from my current project and the results are astonishing! I used very same rendering settings for the same frame changing only the usage of GPUs I have, look at that guys:
RTX 2070 Only: GPU Geometry Setup: 12 s, Rendering time: 77,41 s, Total time: 89,41 s
RTX 3080Ti Only: GPU Geometry Setup: 12 s, Rendering time: 25,96 s, Total time: 37,96 s
Both GPUs combined: GPU Geometry Setup: 24s, Rendering time: 20,97 s,Total time: 44,97 s
Honestely, this is the last thing I expected here. You can see clearly that Keyshot needs to perform GPU geometry setup separately for both cards. For complex scenes, this time is relevant. In my case the benefit of shortening rendering time by having both cards is smaller than cost of GPU Geometry Setup. This is really interesting and quite depressing for me as I have got worst performance by combining two cards, as I had expected to have a boost from that combination.
Unfortunately, I have no access to KS11 as I do not want to spend subscription cost monthly when paying 2k USD around year ago for full license... As far as I know I can't upgrade it right now. Nevertheless, would be great to know what exactly has been improved in KS11 on that field.
To sum up, for very complex scenes I'm gonna use my new RTX3080Ti only, which is quite sad, however, I still appreciate you suggestion very much Niko.
Kuba