Just wanted to respond to this because I just had nearly same situation, same issue. I have the Asus TUF-Gaming x570 Plus motherboard, recently built and had 1 m.2 nvme ssd installed. Everything worked great, but a month later I went to install a second m.2 nvme ssd, system wouldn't boot, and I couldn't get into bios settings. Once I removed that new 2nd m.2 ssd, I could boot and get into bios again. I switched the ssds in the m.2 slots, switched basically all settings in bios relating to pci-e/nvme drives and boot options (CSM, secure boot etc) that could have been relevant, nothing fixed it. I was about ready to throw in the towel, I found this thread and while it wasn't proven the bios update would fix it, I saw OP updated their bios among other things and they got it to work.
I then updated my bios from version 1404 to version 1405 and it worked. Just wanted to post that for anyone else who may run into the issue, it seems to require updating to 1405 to get both m.2 slots to work.
