This question just came up for us yesterday. I like the idea of "tribe" or "clan" for the group, though "pack" might apply to my quad in particular. My other husband and wife apparently refer to us as puppies as a group.
I usually call them my other husband and my wife. Sometimes I call him my Sunday husband because during the last Renaissance Festival he and I wandered around taking care of the kids while his wife and my husband worked, and our kids started calling us Sunday mommy and Sunday daddy. Sometimes I call them my partners. I won't call her my sister wife, because I'm intimate with her and it's just creepy. If I'm not out to someone, I just refer to them as our dearest friends, best friends, closest friends, etc.
I admit I'd like some better names. Since we're married couples with children, we've all agreed that our marriages must be our primary concern, but I don't like the implication that they aren't as important to me when I say "primary" and "secondary".