Meshes in different Parts can't share nodes. This is due to the computational parts approach that Apex uses for solving (you can think of computational parts as kind of like superelements).
If you're creating surface meshes, the way to join them is to use Edge Ties. Internally this will join the meshes using RBARs between coincident nodes. If you export the model to a Nastran deck, you have the option of retaining those RBARs, or ignoring them (in which case you would have to then equivalence in Patran).
If you're doing this with solid meshes, you should use Glue to join them.
