The story, "Video Nasty", is something of a mash-up.
When I was a young man, home from college, I did indeed hang out one evening with two female friends of mine watching a cheesy porn film about conjoined twins. I'll be damned if I can find a copy of it to verify the title. It definitely wasn't Joined at the Clit. Like "Sue and Gabby", my friends were very different in appearance and mannerism but were fairly inseparable for years (though only one of them worked at an independent video store).
Watching porn with a couple of young, attractive women while in the throws of my hormonal distress (from roughly age 13 to age... how old am I now?) was interesting and seems like the opening to a Forum letter. Alas, it was just another "movie night".
The second half of the story is based on some fun I had with two friends years later. Like my other friends, these two young ladies were physical opposites. In this case, however, there was no mistaking that we were all together to have some fun. A lot of what we did together makes up the second half of the story. Ahhh, memories.