Well my friend has, she met a couple through me and kinda found them both severely attractive. I slipped up and the couple were pretty excited about it. They had a fling, they both told me about it. They had fun and also concluded it was best not to see each other again. My feelings on it at the time, I couldn't care less. Kinda awkward.
Another two girls I know went out with this one guy at the same time. They didn't sex, but they would hang out together (they were going out with each other actually, so they were all aware). Didn't work out cuz of the jealousy and attention factor.
teh end.