Maybe LF is onto something. The problem that she is addressing is something that with kids you'd differentiate as "just the same" and "different." The skier who turns the body as a whole is doing "just the same." The one whose legs turn underneath a stable upper body is doing "different" because the upper body is doing one thing and the lower body is doing something else. The ability to do "different" is developmental -- the brain and body need to develop to the point where they can multitask. I'm sure habit plays a role in this as well. (I notice that sedentary older people tend to lose the ability to differentiate between left/right and upper/lower movements: when the left foot moves forward, the whole body moves with it, rather than the left leg swinging forward from the hip while the right arm swings forward from the shoulder...) So perhaps the answer to the riddle of how to teach the legs to turn under a stable upper body is to develop that skier's ability to (ahem) chew gum and walk at the same time.
Here's a great (dryland) exercise to promote what you're after: jump rope while turning your legs right and left. You can't swing the rope and turn the upper body with the legs.
Speed skate: taking a low position, lunge from side to side like a speed skater, driving the opposite arm forward. This trains an active lower body and a stable upper body, as well as cross lateral movements.
One I love from race camp is to do jumping jacks with the legs while punching out with the arms left and right (just the same below and different on top); this trains L/R and upper/lower multitasking.
I'm sure you can come up with many such ways of training your body's ability to juggle while riding a unicycle, or ski.