If you need a really unique and special engagement ring, forget the diamond. A couple would wear a ring each during their engagement. A couple would wear a ring each during their engagement. Go for gems that are rarer than diamonds.
Ring A: Respect, Love, and Loyalty - Ring B: I promise you these. She is not the kind that will stay using the usual wedding ring designs. Let's say you've Ring A and Ring B. During the wedding, the three rings are joined together to become one. Revival of Gimmel Rings.