mac pro – Why will 2012 Mac Pro work with 2 graphics cards, but not one?

I have a 2012 Mac Pro that needs to be upgraded to a metal-compatible card. When I change the cards, the screen shows nothing and I do not think it boots. However, if I insert the original card and the new card, it will work properly and video will work from both cards. The problem is that the Mac does not allow a mojave update with both cards. If I remove the original card, I can go back to the same problem regardless of which x16 slot I use. What could be the cause?