I used this trick on my Tascam's all the time. I had 24 tracks, and never used more than 22 of them. If I got to the point I needed more than 22 or 24 tracks, I made a submix on track 23/24, copied them to a new tape.
If you put the new tape with submix in the first recorder, you've got 22 new blank tracks completely synced to all off your previous tapes/tracks.
It is some extra work, but it's worth the trouble. If you find yourself doing this more than twice a week buy extra recorder units like i did.