We have been together several years and started "swapping" a couple of years ago. We are very informal about it though. If she meets someone she is interesting in being with - fine. If I do, it is fine also. We don't have relationships with others, it is more just friendship and sex. It works for us and keeps us both very horny and satisfied at the same time.
Interesting, I suppose my wife and I were more formal. We didn't play separately, always together. If we met someone interesting we made a threesome of it. That's hoe it worked for us, kept us horny and satisfied too.