Why LVM, when you use BTRFS? Makes no sense. BTRFS can use more than one device. You could simply install Manjaro with BTRFS on one NVME and add the second device. Then you switch the profile to DUP, RAID0/1.
BTRFS filesystems across multiple drives with RAID 0 or 1 (striping or mirroring) has been stable for quite some time. When you’re doing things like RAID 5/6, that is when you would want to do BTRFS over LVM. (Though that won’t be the case in the foreseeable future.)
Not sure if the installer does this for you, but the setup for 2 drives is very simple. e.g.:
mkfs.btrfs -m raid1 /dev/part1 /dev/part2
RAID 1 obviously halves your usable space (while increasing read speed), but one drive can fail. While RAID 0, which might be what you want to do? Would make it so if either drive failed, you would lose everything. (Though you would increase write speed.)
If you wanted root and home on different drives for another reason, that’s just creating 2 different BTRFS filesystems on each drive. Either way, there is no need for LVM in any of these scenarios.