In the fellowship of the ring, Arwen rides a horse sitting in the typical riding position and not sidesaddle, yet she was wearing a skirt. But I thought you couldn't do that. (And yes I did actually wonder about it when I was watching FotR.
The discussion on Arwen's Chase Dress had the answer. The overskirt was constructed from 4 seperate panels that draped as a skirt but could swing apart.
Neat, femme without being restricting.