Hborre gave the answer to the question asked, and I gave the same answer as a pretty picture. The answers are correct, but on rereading your post I think the question may be wrong.

Quote - ... in some cases one arm might be up and the other in another position completly.... so there is my issue.

A morph can only affect one actor (body part), the one it is in. So with two arms you have two morphs, neither of them symmetrical. So presumably the correct question is not "how to split a morph?", but rather "how to split an FBM?". How to do that may be a little more difficult to explain, but I'll try to collect my thoughts on the subject, and get back to you as soon as I can.