As a first approximation: objects and handle graphics created in one session cannot be passed to another session. (There might be mechanisms that I have not noticed yet.)
MATLAB sessions have their own address space, and there is no shared object memory between them, so when the new session gets created the object variable just becomes a reference to something that does not exist in the new address space.
The sharing mechanisms are similar to what would happen in a message-passing system: MATLAB can pass in a "message" which is a data array, and it can receive back a "message" that is the results, but what would be a valid pointer in the original session is just a numeric value in the second.
Only one session has access to graphics: MATLAB graphics are all handled by a single JAVA thread that does not have access to the memory of the other sessions.
It is possible that you might be able to get further using a different kind of parallelism, but I am not certain about that.