I don't know how the order of spouses is determined. I played around a little and couldn't determine it. My best guess is that it is set when you add the partner. You can force both of the partners to one side by adding a third partner and deleting them. But it looks a little odd, so I would make a copy of my family tree and try it there first. I am leaving this as a comment instead of an answer in case someone has a real answer for you.
