It is in the sparkle of the other's eyes, in the warmth of the room when they enter, in the thought they both have and then express at exactly the same time. The third body is especially present when the two are quiet, no words need to be said, yet the room is invariably filled with affection, love, and understanding. Maybe the third body is the combination of the spirit of the two. I don't know. But what I do know is the third body does not appear or make itself known if people are unhappy, unkind, or abusive.
Relationships are hard.
Kindness to self or others is not always easy to come by. But you will find deep gratification in the effort spent attempting to find the third body. When the third body enters the room all is magic and the troubles of life briefly disappear. Stop treating one another as bodies to be had, trophies to be won, items to be gawked at or chided. Relationship is not about hunting, laying, taking, screwing, or victory.