cant you make it so one texture overlaps the other
e.g. have a checkbox for which texture
OR
e.g. choose for them.
and to accomplish this make one half of the arm slightly bigger,enough so no one notices, so it overlaps.
if you feel like you're animations aren't just that creative, then go out and play or eat a burger or something and then come back and work on it again.(doing stuff will get you're creative juices flowing)