Oh, faxxade, it is nothing that money can't fix....
They are saying you need to get 2 same cards, or 2 cards from the same generation.
The thing is that if you look for the newest W7Pro64 drivers for the "lesser" of the two Quadro cards you currently have, at nVidia.com and then look in the associated literature for added supported cards, you often find that the "greater' card is listed as supported under that same driver.
This works great for us with Windows 7 Pro 64-bit.... both the listed older and newer cards run fine with that single driver. But, under W10 the MS auto-install of drivers has been forcing the driver for the "greater" of the two cards on our installs, which can break function of the "lesser" of the two cards. That is, MS is forcing the later driver that works fine for the later card, but may not work properly on the earlier card. Or, it mixes in an older and newer driver, creating a mishmash.
My workaround for W10 is to not mix in two generations.... either use two older cards or two newer cards. For W7 I am able to follow the first rule (paragraph 3) and that works fine.