My thoughts on the whole metamour question.:
It is true that friendships (as with metamour love) neither can nor should be attempted to be forced. And there are many ways in which metamours can relate to one another, from distant near-strangers to deeply bonded friends.
I can only speak personally on this matter. I'd not choose to become loving partners with anyone not very likely
to be highly
compatible as a potential friend (or lover) with my long-time Sweetie. (My boyfriend is not bisexual, so a woman partner of mine would not be a potential lover of his.) I am not interested in a casual outside fling, and really want only committed, "serious" (family-like) bonds. So, naturally, I'd hope my Sweeties (Darlings, Lovies...) would treasure one another, as I do. I'd want us all to enjoy time together at the dinner table, on hikes, camping, picnics, movies, dining out, concerts.... I'd like us all to be sincere friends. None of this can be made to order. It takes time to grow, and the results cannot be planned in advance -- like a shopping trip.
And yes of course we all need time for "just the two of us". And yes we all need time for ourselves, alone--solitude. This we can create together in an atmosphere of loving and friendship. That atmosphere can be nurtured, but not shaped as a sculptor would craft a statue. It feels probable to me that my present new love interest and my longtime partner would get along great and become friends. It's difficult to imagine any other way. Why wouldn't they be?
In the highly, highly unlikely event that they don't enjoy one another's company? I'd enjoy them apart, and be sad about it. (None of us have met yet, face-to-face. "Faraway Sweetie" and I met in this forum and have built a loving friendship with text and phone calls. We plan to meet up soon.)
Ideally, any two or more sweeties of mine would form loving bonds of their own, whether romantic or not.
metamour - http://www.urbandictionary.com/define.php?term=metamour