Haifagirl -“If you ignore the part about other legal stuff (which was parenthetical anyway), that’s what he is suggesting. Two girls become roommates. They each adopt a child. They each have a built-in babysitter.”
The S’A says two women together (Toeiva) is either Ossur M’drabbanim or D’orysa. The S’A also says that two men can be Meyached together -be roomates because Jews aren’t Chosut (accuse)on Toieva. This would be the same for two women. Once you go beyond the boundry of two people just being roomates, like for example this case, two women acting as parents, even if right now they aren’t doing anything Ossur, I see no reason how Yichud would be Mutter. You are putting two people in a entirely different situation than roomates, they are having a very intimate relationship -raising a child together. This would definitely be Ossur because of Yichud!