Can I use two things like LVM to glue two discrete partitions on the same flash into one partition?

I don't want to adjust the partition table.

You can convert each partition into a physical volume, add then into a new volume group, and then create one or more logical volumes inside... Ten times the work required to redo the partition table, and you will also lose all your data.

use btrfs and stripe the two partitions into a single filesystem. You'll need to copy all the data off, and then copy it back once the new filesystem is made, but it will work well.

This doesn't look so complicated, is it suitable for use on the router's ROM?
As I said, the partitioning of the original firmware of this router is very strange.
It has two 19MB partitions for the Linux kernel. A 70MB partition for the user configuration.
The partitions are not continuous.

BTRFS does not seem to be suitable for use on the router ROM.

Why not? It has specific knowledge of flash devices. And it has a mode where a static FS forms the basis (seed) of a filesystem where updates are then placed elsewhere... It's a pretty sophisticated FS.

Your biggest issue is you seem to want to flash an image onto the device instead of building the FS on the device?, in which case I don't understand at all, just nuke the partition table and write a sane image to the device

Sorry, I thought you meant a "flash drive" not the "internal flash" from the device! Forget what I said, I would not touch the partitions on the internal flash, unless you want to reconfigure and rebuild the whole firmware yourself.

Btrfs uses a portion of the disk space to store metadata. This is too large for ROM flash, and the gzip and zstd used by btrfs do not provide a higher compression ratio than lzma.