I'm trying to find a series of short stories where either Sam and/or Dean spends a night with one of the Buffy girls, either Faith or Dawn and I think Buffy is here too.  The morning-afters are rather abrupt and end quickly.  For instance, early one morning Winchester is awaken by Xander banging on the motel door, yelling for Dawn to get up and make ready to hit the road, he's got coffee and Buffy'll kill them if they're late, or something like that and she throws on her clothes and runs out the door with a quick good-bye.  Another time it's Faith and maybe the other brother.  Slowly, over the course of the series, Dean and Sam realize that these girls they're running into in different places all know each other and the girl's discover along the way the guys are brothers.  I've checked Twisting the Hellmouth but cant seem to find it.