Tried something like this for a moment thinking I could lessen the frame rate drop when popping out map and ATC to second monitor. In my case it was two nVidia cards, a 960 and 1070. It seemed to me that one card still did all the work and no help. It was like the main card was still paining the render, the second card was just holding it up and showing it to you as in no real help.
This may be of help if you are not trying to use the other monitors to involve FS2020 and would just like them to do entirely other things?
I did not add much technically other than getting both cards to boot together, I run a Corsair CX750M and it did not catch fire. 