You can use the Poser 7 morph tool to morph vertices of a child actor away from those of the parent, at the weld seam.  The result can be ugly rendering artifacts, where this has happened.  It doesn't look like Poser averages the positions, in such cases or, if it does, the results are unpleasant.  It looks like the parent vertex at a weld is dominant, setting the position.  But the child vertex's position apparently contributes somehow to normals calculations at render time.

You can sort of test the parent-child morph behavior at welds by welding two identical geometries, so they overlap completely and end up with welds for every vertex.  In such a case, both actors will respond to deformations only of the parent actor.  Morphs in the child will be deactivated unless "Bend" is turned off for that child.  This seems to verify that the parent vertex at a weld will dominate.

I'm not sure how things might sort out in the case of "illegal" welds, between two actors which don't have a parent-child relationship....  :unsure:

Edit:

Thinking about this, it probably relates less to the actual actor-level parent-child relationship, than the way the "weld" listings are set up in the .cr2.  There's an implicit parent-child relationship set up there, regardless of where the actual actors may fall within the figure hierarchy.

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Cage can be an opinionated jerk who posts without thinking.  He apologizes for this.  He's honestly not trying to be a turkeyhead.

Cage had some freebies, compatible with Poser 11 and below.  His Python scripts were saved at archive.org, along with the rest of the Morphography site, where they were hosted.