I think we need to fix booting with both your cards installed and not in safe mode. When you mentioned that you don't have any errors when booting verbose, what happens when you boot verbose, is it Clover that complains and not macOS?
Ok, this was the one of the reasons I wanted to see both monitors connected individually to each card with the iMac 17,1 system definition. I wanted to if your first card was GFX0 and your 2nd was GFX1 as this can be affected by Apples display policy. You could test using a MacPro system definition but you need to take the necessary precautions to eliminate kernel panic at boot. Or test using iMac 14,2 that bypasses AGDP.
Unfortunately nv_disable=1 is booting in safe mode for Nvidia graphics cards, booting in safe mode is all drivers such as audio, graphics etc. For now I would suggest backing up your config.plist since you're happy with iMac 17,1 and change it for testing to iMac14,2. Since we are unable to get a workaround with graphics enabled at this point this could eliminate any issues with iMac17,1 and its easy to revert to your current config.plist.
On the other hand, I really would like to keep 17,1 as I got everything working incl. Messages.
If you've registered this system/variables and its sounds like you have with Apple Servers then I wouldn't advise to use them with a different Serial (it may be ok but no guarantees). This would surely flag your AppleID and could cause an issue with it.
Understood, Unfortunately the only difference using these system definitions is AppleGraphicsDisplayPolicy. I'm pretty sure that a second card would register as GFX1 which is noted in the kext info.plist. At this time the only possible idea I could see is to rename your second graphics card entry currently named PEGP. See this thread for example https://www.tonymacx86.com/threads/...imac-15-or-imac-17-system-definition.183113/s. You can use Clover's DSDT patching for renaming and testing.
