[First, if I have .pz3 files for two different images, is there a way to import one into the other to sort of merge them?  I've got files for two different characters I've rendered in separate files and want to do an image with both characters in the same image/file.  Is there an easy way to accomplish this, or do I need to start with one and re-create the other within that file? (hope that made sense)]

Load your first .pz3 file into Poser.
Then from the menu bar click File:Import:PoserDocument/Prop
and load in your second .pz3