I read that this is a rather uncommon mutation, an...

That's a pretty awesome mutation! If you purchased the seed as fem they should be female. The chance of a true fem seed growing into a male is like 1 in 1000 or possibly even rarer than that. The reason for the differences in growth could be a couple of things. 1 would be that even though they are twins they may not be identical. That would mean they would likely grow at different rates as you are experiencing and they will have features unique from each other such as leaf shape, bud shape, and internodal spacing. 2, you may have stunted the one that's growing slower by doing the separation. If you happened to damage the root of the smaller one a little when you separated them it would have taken the plant a little bit of time to recover before it could resume growing full speed again.