First step is to calculate the convex hull. Assume your points on the convex hull H are p0, p1, p2, p3, ... , pn, p0. Let us assume that the optimal convext hulls A and B contain subsets pA and pB from this list. Then pA and pB are obtained by splitting H at two points.
Now you can see that you can splits points on H only O(n^2) ways.
Since you do not want complete answer, I am leaving rest to you.