I've been wondering the same thing myself, kind of. I don't believe in souls, fate, or destiny of any kind, but I certainly have an unutterable respect for the bond between my husband and myself, forged in fire for nearly 14 years now.
We've been talking about a triad with a close, dear friend, whom we met at the same time as each other, and who has been an important part of our life and an officiant at our wedding. I love her wildly and deeply, as you said about your second, but wonder if I would ever come to feel for her with the depth and trust that I feel for my husband. I love the idea of an equilateral triangle, but I'm not sure it's possible for us. I'm not closed to the idea, I'm just truly unsure. I want it very much right now, but I'm also trying to be cautious. You don't carelessly rock the boat on something that has been your rock for your entire adult life. And I wouldn't want for our friend to always be the "second" in her relationships with us or anyone. That might be okay at first, but it wouldn't be right forever. I want for her what I have with my Bear.