From "back in the day" PornStars Joanna Storm ("Devil & Miss Jones #2" and "Firestorm") and Sharon Mitchell were reported to be a real life couple. Both women did b/g as well as g/g in their sexvids. Also, Janine has given interviews saying that, going back to their pre-porn days, she and Julia Ann use to fuck each other.

Of the "newer" PornStars, it seems that although most have boyfriends, the girls also have off screen g/g relationships. The recent (ugly) pictures that turned up on websites a couple of months ago of Tawny Roberts & Mary Carey fucking each other in Florida is evidence of this reality.

But, to get to the other part of this question: the 1990s sexvids "Things Change: My First Time" & "Things Change: Letting Go" starring Nikki Dial and Deidre Holland speaks to the theme of a Lesbian relationship trying to survive after one of the partners (Nikki) decides she wants to "experiment" with b/g sex. Oddly enough this is the same theme that's playing out on the current Showtime lesbian themed show, "The L Word."

Hope this helps.