In high school I knew identical twin girls who shared a boyfriend. He was convinced that he was dating only one of them -- they even had synchronized their menstrual cycles, and kept the same social calendar. The one who was "officially" his girlfriend would let her twin sister "sub" for her when she felt like taking a night off but didn't want to disappoint her beau. It got somewhat more complicated as time went on, of course...
They let him in on the fact that they were tag-teaming him eventually. He dealt with it well, I think -- he asked them if they would consent to both being his girlfriends, in the open. The girls decided that since he was a nice guy, and obviously cared for them, they'd go along with that as an experiment (this was the 60's, after all).
They were very pretty girls, too. So he had a blast...
Last I heard they were still at it, but that was 1970. I have no idea how it worked out eventually.
Dated ONE twin once. Walked up to her sister and said, "Hey, I called you last night but you didn't answer."
Everything seemed to all go downhill from there.