So I ran downstairs and grabbed a roll of duct tape. And we duct taped Ava and Owen together at the arms over their sweaters, and around the calves over their pants. We explained to them about how we should never wish the other person dead and that they would miss each other terribly, and because they couldn't get along they now had to be forced to be together and figure out how to make it work.
They overheard me telling Chris that I thought they should stay taped together when I went to someone's house for a couple of hours. This sent them into a fit of crying that pretty much lasted as long as they were taped together. (About 40 minutes.) We made them fold laundry and pick up stuff around the house as one unit.
I almost couldn't handle watching them! I had to leave the room and laugh. There is a good chance that this punishment is going to stick with them and be a good reminder as demonstrated in the following picture: