I'm running a Ryzen 7 7800X3D on an Asus ProArt X870E-Creator WiFi motherboard with 64GB of DDR5-6000 memory and a Corsair RM1000x (2024) power supply. I previously used an Asus Prime RX 9070 XT alongside an Asus ProArt RTX 4070 Super without problems. While upgrading to a second RX 9070 XT for better Linux support and more VRAM, I found that either Prime card works with an ASRock Steel Legend RX 9070 XT, but the two Asus Prime cards would not appear together. Only one GPU was visible during POST and in operating-system tools, regardless of which PCIe slot or card I used. Both cards have the same VBIOS version, and each works normally by itself. I wondered whether the 3x8-pin power requirements, a splitter cable, or the 1000W PSU could be responsible. After removing the system from under my desk and opening the case on a table, I discovered that the motherboard's plastic edge shroud was obstructing the cards and preventing one from seating properly. Reseating both cards correctly fixed the problem.
2 Answers
A splitter can be worth investigating, since a card may fail to initialize even when its expected maximum power draw seems within the cable’s limits. Try running the second card with a dedicated connector arrangement where possible, and test a card combination that previously failed. If the system still works with the splitter, the cable probably isn’t the cause.
Before replacing the power supply, test each card in both PCIe slots by itself, then try both cards together with the exact same slot arrangement. That helps separate a bad GPU, slot, seating issue, and power problem. If either card works in either slot alone, the cards and slots are probably fine, so check for anything physically preventing one card from fully seating.
That suggestion got me to take another careful look once I was rested. The case’s plastic motherboard shroud was interfering with the cards, so one of them wasn’t seating correctly. I opened everything up on a table and reseated them, and both GPUs appeared normally.
I tested the cards individually and with the splitter already, and those combinations worked. The actual problem was much simpler: the case shroud was getting in the way of proper installation. Once I removed that obstruction and reseated both cards, everything worked.