Poking around in the system I found that card 0 was not running at all, so I restarted the system and it now seems stable for GFN 15 on both cards.
Perhaps one card not working caused the other to fail calculations although that is beyond my understanding as to why it would.
I don't OC but took a measured approach and turned down the voltage and clock on card 0. Card 0 being on the top always runs hotter but below the point that it should matter.
If the system continues to run reliably I will attempt a GFN 16 and then GFN 17.