As a platformer, the controls and the feeling of controling these babies felt fine. Dragging the other counterpart around didn't greatly hamper the feeling of movement, instead adding to it when they're swinging with you.
For the Q/E skin growth/shrinkage mechanic, I would suggest to make it a hold down control rather than an on press control - pressing either once doesn't feel like anything's happening, since there's no instant feedback.
I think it's also rather important to have an indicator of when each twin can interact with the world - something like a climbing animation state for the boy and the skin being outlined as the girl. Shift as a swapper would better be replaced with space or f - something closer.
A bit of a morbid theme... the needle balls and the death sounds definitely were a little... off. I enjoyed it overall. I ended up throwing in the towel at the first level with a pill - I couldn't bear to die one more time.