Take the top of the pump apart as Olav suggests. Perhaps the suction on the pump out was too strong or a partially clogged vent line caused the rectangular rubber valve on top to distort. This may explain why both units went down. Had to replace that same rubber valve a few years back but I was having problems pumping unit where it was air locking. Good luck. Toilet rebuilds are worst boat job I can think of.

David

Thanks to all for your inputs. Turns out the reason both heads failed at the same time wasn't becuase of the pumpout, but rather the very silty water in Seward Harbor at the time. Both of the head gaskets were coated with a slimy slit that was blocking them from operating properly.
A simple cleaning with a paper towel and all is well.