One thing I keep in mind with double-fall-line trails is that it's very difficult to ski straight down one path. i.e., if the trail is 50 yards wide, then keeping one set of trees 10 yards off your shoulder is rather hard to do. Said another way, unless you are very deliberate, you will get "pulled" to the "lower" side of the trail.
I tend to not really fight that. You can always throw in an occassional larger radius turn that resets your line. i.e., if the double-fall-line is pulling you to your right, then occassionally throw in a larger radius turn to your left to get "back".
One thing that I've noticed that seems to mess with a lot of people's minds is that on double-fall-line trails one turn direction has the potential to accelerate you. i.e., you turn into one fall line, and as you turn out of that fall line, you're turning into the other. Conversely, the other direction can turn you out of fall lines, and speed starts getting scrubbed in a hurry -- you're just going uphill!
You can practice on single-fall-line trails by just varying turn shape. i..e, right turns have one turn shape (say, short radius), left turns have another (medium radius).
As JohnH said, skiing with the same turn radius on double-fall-line trails can be done and is highly educational, but it requires being very conscious of what you're doing. Mixing up turn radius' will likewise do wonders for your skiing and route-finding skills.