i'd avoid dating conjoined boyfriends if i were them. Imagine if 1 relationship is not working out and another is great. With 2 diffrent guys, the one with bad relationship can just move on and find another guy. But if their boyfriends were also conjoined, she can never avoid her ex-boyfriend since her sister is still dating her ex-boyfriend's conjoined twin.
Imagine the problems, if all of them were married or living together! She can't move out and she can't kick him out either!