Friends of mine have this type of relationship. They each have 2 or 3 "others". This started in their marriage because the wife is Bi, and she was missing the physical contact with females. They both know the other ones "others", and there always has to be prior consent before they do anything. They call it Playtime. They don't venture beyond those extra 2 or 3 people. Sometimes one will leave, and they will find someone else, but they always meet and get to know the person. She will plan a date with someone after making sure it's ok with him, and then be on her way, and he will do the same.
They are completely happy with each other, and have a great marriage.