I don’t really see them as ultimate enemies because it seems like they are working together, using each other. They do try to attack each other, but almost halfheartedly. They always seem to really have some sort of other plan going on.
It depends on what happens towards the end of the season. It’s quite obvious that their “alliance” is on the cusp of disaster. Judging by the promo, the deal between Mr. Gold and Regina has deteriorated. So, I think they’ll be on completely opposite sides by the end of the season.