Yeah, it's got some of the same weaknesses as multiprocessing (and several new ones). Conceivably you could provide an API for handing off objects to the other interpreter without copying. I'm imagining an API like:

  my_foo = interpreterX.pass_object(my_foo)

(The assignment being required to delete the originating reference from the source interpreter.) The interface would be obligated to check that there are no references that escape to the current interpreter and then my_foo and all referenced objects could be handed off to the other interpreter in whole.

I don't have any intuitions for if that would be cheaper than copying or not, and getting it right is certainly more difficult than serialization. (Because of the complexity, it's not worth having if it isn't cheaper.)