Hard to say, I presume each card is driving four monitors? If one card is able to drive all monitors at native resolutions and other is having issues driving one of monitors at native resolutions. It would role a simple bandwidth issue on the GPU.
The 4x PCIe slot should be able to provide enough power and bandwidth for the second T-600 otherwise it would have not booted-up on POST.
Gigabyte Z390 AORUS-PRO Coffee Lake R 9700K, 2x16GiB of G.Skill RIPSAW DDR4-3600, Sapphire RX 6900XT, Seasonic GX-850 and Fractal Define 7 (W)
Ivy Bridge 3570K, 2x4GiB of G.Skill RIPSAW DDR3-1600, Gigabyte Z77X-UD3H, Corsair CX-750M V2, and PC-7B