Combine GPU calculation power

After Windows 10 Fall Update you see all of your GPUs in the Taskmanager:

When I play a game, only one GPU is used, the other one is idle.

Question: When I reach 100% with my GeForce GTX 760 is it possible to use the unused power of my Intel HD Graphics to get a few more fps - even if there is no monitor attached to it?

Or Gaming on the one GPU, video transcoding on the other one.

Short answer is: "No, you cannot combine Nvidia, AMD, Intel, etc different vendor GPUs".

Longer answer is that you can combine two or more GPUs from the same manufacturer if they are supported.

Ie. you can use two Nvidia GeForce 1080 cards in SLI mode or two AMD R9 290X cards togethe, etc. In past you could even use two different model cards; AMD HD 5970 and HD 5870, for example - and it would default to the speed of the slower one.

Using two cards in Windows is tricky and requires vendor drivers, support for specific games (or you're just as likely to run slower than when using just one card), etc.