Can't cheer you up a lot, because :With my R9 5950x, exactly one DIMM works, as soon as I populate the second channel, I get code 00 on the Dr. Debug LEDs and no post.
I had similar case with X470D4U. Tried different CPUs, different memories, different configuration - just refuses to boot with 2 DIMMs. So I gave up.
I just assumed the board is buggy (its technically not DOA, as it was working) and moved on to X570D4U.
Its still hard for me to trace what could go wrong - apart of bad power/mosfet control (not being able to power up more than one DIMM at a time) or bad BIOS (which I dont see how can affect different RAM slots), I don't see what can cause this issue as the RAM is directly connected to the CPU and does not "talk" to any other parts of the MB