Dick, I don't know about gluing your sanding belts back to together,
but I would say they are failing at the joint because the belt tension
is too great.  I've been using 6" wide belts for sanding my ocarinas
and ocarina parts for well over 20 years now, and rarely have I had
a belt fail at the joint.

Your narrower belts have less glued surface, so they are inherently
weaker at the joint than the 6" belts, but they should hold up for
a long time with the correct tension applied to them.  Try adjusting
the tension of your sander down as low as you can without slippage
and tracking problems.  The belt should run until it's worn out.

Charlie Hind (Honea Path, SC)
http://www.hindocarina.com/