Who’s to say it isn’t already?

Actually if the pee is distributed throughout both balls, then it is stored in the union of the balls, i.e. all the space in one or both balls by definition.

Furthermore, the intersection as depicted in the diagram only includes the area common to both balls. If we take the “balls” (sets) to be the testes themselves, then anatomically the pee is stored nowhere because the two testes are physically separated and therefore have an empty intersection.