ex-swap
Usually occurs between two ex couples whom the guys are bros are the girls are tight. When things turn south in both relationships, the bros and girls don't mind hooking up with the other guy/girl. Everyone benefits because both the girls and guys feel great being on the rebound while simultaneously getting revenge by hooking up with their ex's best friend.
Remember Janet and Charley used to go on double dates with Kate and Josh? Yeah well now Charley's doin Kate, and Josh is doin Janet. Classic ex-swap.