Is there some way to edit just one side internal lines in a symmetrical pattern?
I think there is, but I can't remember how it is done.
You cannot edit an Internal Line on one side of a Symmetric Pattern without removing symmetry first. If you want to edit one side and not the other you must first remove symmetry. Many users make sure to do as much work as possible with symmetry before they remove later.
But you may be thinking of remove Linked Editing for Topstitching which can be done.0