In the current game the way it works, this scenario is fairly standard
I am a facist as well as Peter and Ghandi. They both like me because I am peaceful and diplomacy aware and also see the benefit of a political union.Peter and Ghandi hate each other because they are always declaring wars.
Pop on a civ in your current game and look at their relationship with everyone, normally hate abounds. I cannot see them sharing breakfast at Tiffany's
But OK in theory, what reward for such a union does one suggest?