My two-piece is stapled together too. To replace my water pump, I simply removed the bolts from upper half and used pliers to remove the staples. To put back in, i simply drilled a small hole and put a screw in it where staples were to keep the halves together at the correct spot.
But, to get to proportioning valves, the bottom half must be removed....as mentioned, might as well remove the entire shebang as one piece.