Specifically, Mint can't seem to decide which NVMe device it's installed on. It varies from one system start-up to the next. After one boot, the Preferences > Disk utility shows the Iinux root partition designated as /dev/nvme0n1p1 (i.e., 1st partition on the nvme0n1 device). After another boot (or several) the same utility shows the root partition designated as /dev/nvme1n1p1 (i.e., 1st partition on the nvme1n1 device). As the UUID remains consistent the boot process itself is unaffected. Nor does there appears to be any adverse impact on anything else. But surely that kind of "switching" behaviour can't be considered as "normal"! Can it? I dread to think of the consequences of inadvertently choosing the wrong one for repairs or updating.
If anyone else has a similar dual NVMe set-up and/or has any special insight into their usual behaviours with various NVMe M.2 installations, I'd greatly appreciate any feedback. Although there are no actual boot problems, I have used boot-repair to generated a complete system report in "Mint Live" and I can post that if anyone wants to see it. It's a biggie showing no obvious issues, but I can make it available on line if thought likely to be helpful in any way. Its only "final advice" is as follows:
In fact, my UEFI-BIOS has no problem detecting and using Mint's boot files and so I've taken no action on that suggestion.The boot files of [Linux Mint 19.1 Tessa] are far from the start of the disk. Your BIOS may not detect them. You may want to retry after creating a /boot partition (EXT4, >200MB, start of the disk). This can be performed via tools such as gParted. Then select this partition via the [Separate /boot partition:] option of [Boot Repair]. (https://help.ubuntu.com/community/BootPartition)