This works really well on small toothed belts.
I built a roller for my maslab robot out of scrapped printer parts, but the belt went missing. I borrowed one from another team, but it was about an inch too long.
So what do you do?
Step 1: The Answer!
Heat shrink tubing! Hooray!
I added a daub of hot glue to the smooth side of both belts first.
Then I slipped a bit of heat shrink around, and hair-blow-dried that belt like I was a stylist at a posh salon in New York.
It works really well! It acts just like a continuous belt, goes around the gears just fine, doesn't seem to have any elasticity or want to pull apart.
Also, the heat shrink tube goes all the way around the belt, and grabs the teeth, so it's better than any glue solution (and it's compatible).