z90 xtreme

Got a 13900k and a Z790 xtreme. Running 2 sticks of 32gb corsair dominator ddr5. With both sticks in slots A2 and B2, get motherboard code 12. Won't post. With a single stick in B2, it posts. Both sticks work by themselves in B2 slot. Neither stick works alone in A2. Assuming this has to be a...