What kind of glue can be used to join flat drive belt and v-belt?
I'd use Shoe Goo ... it dries rubbery and flexible.

Another alternative would be contact cemement (I use DAP, personally, do not get the non-flamable one that cleans up with water), but it's much harder to work with in general.  And they're serious about using it outside -- the fumes are very strong.

And my last choice would be basic rubber cement ... the bond's not great, but if you're reinforcing it, it might be okay.  (just don't get the 'repositionable' stuff)