Definitely looks like two separate plants to me. I'm not too sure there is anything you can do to straighten it out other than cutting it off and rerooting it. Separating the plants wouldn't be too traumatizing, considering you can store them bare root over the winter. Plumeria are pretty resilient with these types of things. I personally, would cut the branch off and root it. I would also separate that trunk into its own pot. The worst that could happen is the branch doesn't root, and you have to wait for the new branches to form on the trunk. Back to waiting for blooms, but fixes your issue. The may start choking each other out in that pot together, anyway.