pvc glue is the stuff you brush on plastic plumbing when you join it. It even creates a heat when it's working.

How's this for a new angle? Are you glueing the pick to the INSIDE of the thumb pick or the OUTSIDE? It seems as though there might be a mechanical advantage to the inside, as the thumb pick would be holding the pick against your thumb instead of getting pried off the surface of the thumbpick with each pluck/strum/attack.

clear as mud?

Sure the thumb pick's stub would still stick out a little ahead of the flatpick, but it could be polished almost smooth. Isn't that what God invented Dremel tools for?