I can't make both cards work properly on Windows 10. Here's what I've tried.
I've got a successful Windows 10 installation over my Mac Pro (mid 2010) over Sierra, installed the boot camp drivers as recommended on this site. Everything is perfect.
My monitor is hooked to the GT 120 (to get access to EFI boot screen), Windows couldn't find the driver for it and found the GTX 980ti automatically. After the reboot request I could notice it wasn't working properly, the resolution screen was ok but you couldn't open any windows, they got stuck on the bottom bar and the icons beside the clock were missing.
Through the device manager, I uninstalled the GTX 980ti, asked for Windows to update the GT120 driver, it worked like a charm. After reboot Windows, it found automatically again GTX 980ti, installed it without asking for permission and requested to boot. Again, after booting we returned to the ground zero, GT120 didn't load the driver, and GTX 980ti remained with the same issue.
I'm not the Windows guy, so I do not even know if that is possible.
Please, any help would be welcome.