I recently bought a (pair of) TUF gaming X670E-Plus motherboards for a couple of upgrades. I have two computers with exactly the same components: RAM, CPU, Motherboard, coolers, etc., and one of the computers refuses to POST if I have a second stick of RAM in slot A2 (for dual-channel) and the yellow DRAM-light stays on for hours (I thought maybe it took a long time to train, but that's also not the case). However, the computer works perfectly fine with only one stick in B2 and boots instantly. My other computer has no issues whatsoever and boots fine and runs EXPO profile with no issue.
I have tried reseating RAM, trying multiple slots, updating BIOS, clearing CMOS, removing the GPU (just in case), and all still had the same problems.
Any other ideas on how to approach solving this before attempting RMA?
That can be a symptom of a bad CPU mount. Remove the CPU, carefully inspect the CPU and socket for any signs of contamination or damage (misaligned pins in the socket, etc). The socket pins are extremely delicate, and easy to damage, don't poke at them. Carefully reinstall the CPU.
When mounting the cooler, ensure the pressure is evenly applied and not excessive. Too much cooler mounting pressure can distort things and cause bad pin contact. With some coolers, as long as you are using the correct AM5 mounting kit, the securing screws/nuts are just tightened until they stop, but others should not be cranked down hard if they get tight before running out of threads.
Before remounting the CPU, try the stick that's not working as a single stick in B2, to confirm that both RAM sticks are good. If both sticks work individually in B2, the problem is likely the board or CPU (including possibly being badly mounted).