(Just creating more options.)
Instantly, you feel different. Did it work? Glancing around, you notice that your height has decreased, and the costume now fits oddly over your body. On the other hand, you still feel a bit off. Also, you seem to be alone. Where is Alex?
“Where’s John?” You hear Alex ask, though his voice still sounds female, perhaps a little lighter than before but still a girl’s pitch. Wait, does that mean-
“Oh, your mother’s definitely going to kill me.” Uncle Jack stumbles back and sinks into a chair.
“We’re still a girl!” Alex finally realizes, holding her slender hands in front of her face. “But we’re not the same. What’s going on?”
“A lot of things, really. First of all, why did you have to choose a female costume?”
“John picked it. It was supposed to be a joke, I guess.”
Uncle Jack pinches the bridge of his nose. “Well, I have bad news for you, both of you. This particular female costume was never meant to be worn by a male, let alone two males. This means you are stuck as a female. Not only that, but sexual activity in your combined form really messed with the spells. You see, there’s always a chance a woman will have sex while wearing one of our costumes, especially ones so physically focused like this one. So, to avoid complications, whatever form she takes, any sexual activity - even with herself - will trigger a safety feature to temporarily revert her reproductive system to that of her normal self to prevent unintentional hybridization or magical defects in any resulting children.”
Seeing that his long explanation is overwhelming his audience, Uncle Jack brings the topic home. “So, when you two, er, stimulated yourself, this triggered the spell, reverting your female reproductive system to what it thought would be its original state, not knowing that for two teenage boys, there was no such original state. In order to do this, the spell had to treat you like a single person, averaging out your separate aspects and flat-out filling in the rest itself. The result is…well…you’re stuck together. Like this.” He gestures over his nephews, now a composite niece.
“So, what do I look like?” Alex asks.
Her uncle points to a full-length mirror in the corner. “See for yourself. And don’t worry about taking off the costume. I have a few calls to make anyway. Just tell me when you’re finished.” With that, he leaves the room.
Quickly, Alex swings herself off the table and rushes over to a mirror in the corner, shedding her oversized costume for a first glimpse of what haywire magic had wrought.