Same-dating occurs when the two individuals in a couple each have dated an ex-girlfriend/ex-boyfriend who are now currently dating each other. When this occurs, each member (of the two couples) has same-dated every other member in the group.
Guy: "Hi honey, did you hear that my ex-girlfriend, Stacy is dating your ex-boyfriend Kevin?"
Girl: "Ewwww! I can't believe I same-dated Stacy!"