Its been a while since I taught but I thought it was important to teach this in context of what the student was already doing so that progressing from wedge turn to wedge christy to open parallel was just a continuous flow of development. I think instructors have a bad habit of using drills to isolate movements and skills instead of just teaching skiing. I'm not saying such drills are not useful, its just that these progressions are so smooth and natural especially at these levels that it seems unnecessary and maybe counter productive to pull apart the components and teach them in a isolated fashion. The skills and movements have to be blended and what better way to accomplish that then to learn them in the context of the turns? Wedge to wedge christy, for example, nothing really changes, the student just increases speed as he or she becomes increasingly comfortable with making nice rounded, controlled wedge turns. Moving the body toward the inside of the turn as centrifugal force is experienced is quite a natural movement and, with a little guidance and encouragement the student confidently begins to achieve a wedge christy. The student isn't aware of any potentially intimidating or challenging new task just an increasing refinement of what he/she had been doing. All of the basic movements and skills were already present, albeit possibly to an almost imperceptible degree in the wedge turn. I'm skipping over a bit here. I assume the instructor understands the significance of leg rotation for example, turn shape and proper turn initiation and knows what a good wedge turn is.
The issue I have with trying to teach this as exercises is that the student's progress is slowed and the student is made to feel as if there is a significant new hurdle to be overcome which is what typically impedes development.
The student in any case does have to be observed continually and coached to keep him/her on track, correcting any bad habits as they begin to appear.The basic movement is simply to extend one leg and flex the other. This is in effect what the student does when moving toward the inside of the turn. You want to encourage that and encourage the student to amplify the movement a bit. The student should be allowed to discover the effect of doing this in the context of a turn. Let the student's wedge simply go away.Hopefully you haven't been emphasizing the wedge form since the student's introduction to it. The wedge is only a way station and frankly, if the student can balance, turn his/her skis in a controlled round turn it doesn't matter if the skis are wedged or not. As a student becomes increasing comfortable with flexing and extending in the context of the turn the wedge christie becomes an open parallel. The student discovers simultaneous edging with a little coaching. All of the components ought to be present to some degree in the student's very first wedge turns so it is merely a matter of refinement and development, hopefully guided by a knowledgeable and skilled teacher.
I should add that with children the progression can differ a bit. I used to actually teach simultaneous edging as "gripping" in the context of a traverse between turns in order to cause them to develop a sense of confidence and security in parallel edging. Give them a few assists like this and take their minds off of the subject a bit and children can learn faster than you can imagine.