Its sibling welding I think. In my own experiments, like you say, I was unable to reproduce this with the setup room. But when hacking the weld commands in the cr2, poser accepted it. and you can do awful things there.  finding out which vertices are supposed to be the same, gets a quite tricky question.

 

    weld    lMid0:8  
             lHand:8
    weld    lMid0:8  
             lIndex0:8

this is from roxie. its cross-welding the hand child actors. (lMid and lIndex should be siblings)