The first thing that comes to mind is a standard epoxy compound -- but more to the tune of fiberglass epoxy and not the 5-minute stuff.
You'd have to cover the entire belt for about 1/2 an inch on either side of the join. Then let it harden in a semi-cool room overnight and it should stick together ok while maintaining flexibility. We did something similar to this for a sample of our
conveyor belt material and it's working out ok for our (low torque) needs.