that is a twisty one. my opinion: supposing one wants to spend all of eternity with someone, the right one, all other things being equal, is simply the one that returns that desire. if both do, then the right one is the one that helps one improve oneself- spiritually, emotionally, and mentally. if both do, then the right one is the one that cant be said goodbye to bc if this point has been reached with two individuals, risk of regrets/discontent/disloyalty might be too great to continue any type of relationship with both.