https://forum.foldingathome.org/viewtopic.php?t=43104 may help if you are talking about Windows 11 (and probably W10 also) and currently supported GPUs. The key point is that both GPUs must be able to use the same driver.
I also had a more recent experience last week with an old Z800 /Quadro 5000 /Win10 combination (User Z810) that I decided to fold to see just how much it would cost me to run an old Xeon X5667 (4C 8T) CPU in Folding. The answer was "a ridiculous amount of money" so I decided to add a spare Quadro P1000 and Quadro K1200 just to help it reach "Quick Return Bonus" status before I switched it off or went bankrupt, whichever came first. Note that the Quadro 5000 is no longer supported for folding and I was running Fah 7.6.21.
With the Quadro 5000 drivers neither of the new GPUs were accepted and showed only as disabled. [The process also generated 9 "expired" WUs and thus defeated the aim completely, since you need fewer than 10% failures to get QRB]. With a driver suitable for the P1000 and K1200, they both worked OK but the Q5000 (which was the only one with a screen connected since I did not have a mini-DP/DVI lead spare) ran with a basic Microsoft Driver that didn't support full HD, but was adequate for basic operations. So experimentation is the way to go, but watch out for failures that impact you completion rate.
