Merge Time Machine Backup
I tried: it works.

Here is how:

I have a time machine volume and a backup of it. I update this backup every two months, using Super Duper.

Recently, several successive mistakes made my time machine volume loose its first two years of backups. I then had a primary time machine volume with all backups but first two years and a backup time machine volume with all backups but latest two months.

I decided to experiment. Starting from scratch on a new hard drive, I copied the backup time machine volume to the new hard drive with Super Duper (backup all files using erase and save option). Then I copied the primary time machine volume on top of that, again with Super Duper (backup all files, using copy files that are different).

I ended with a new working time machine volume including all backups, from day one up to today.