Poser will allow switching between any geometry you like, and applies the correct morph according to the vertex count of the geometry that's loaded. If the same morph is to apply to more than one switched geometry, they do have to be "morph compatible" of course - i.e. have the same vertex count and order. I think this also implies the reverse - that you can only have one morph set per vertex count, but I haven't tested it.

To put that another way: say you have two heads you can switch between. One has 1000 vertices, the other has 1500. You can have two independent morph sets, one intended for a 1000 vertex mesh, and one for a 1500 vertex mesh. Poser will automatically make the right morphs available based on the vertex count. If you want to introduce a third head which isn't morph compatible with the first two, it must also have a different vertex count. If your third head has 1500 vertices, it will have the 1500 vertex morph set applied to it, so it must be morph compatible with the other 1500 vertex head.

Unfortunately you can't have non-coincident vertices at the boundary between two groups, since this won't permit welding to take place, and would make it impossible to change geometry resolution for individual body parts where there's a smooth join. You could probably do it for the eyes, fwiw.