1) pepperoni on left and mushrooms on right
2) mushrooms on left and pepperoni on right
So with this assumption, your answer is too high and you would have to make some modifications.
Edit: One other thing, it's not clear whether it's allowed to choose no toppings on a side. If this is not allowed, we would replace 2^9 with 2^9 - 1.
Edit 2: Dang I really need to learn how to read. We have to disallow the possibility that the two sides are the same, by the problem statement. So assuming that it's allowed to choose no toppings on a side, and assuming that (1) and (2) above are actually distinct, the answer would be (3)(2^9)(2^9-1).