I certainly don't speak for him, but Bill Ellis uses two hands in his CD, and his edges are as good if not better than any I've shaved with.
I actually find it easier to use negative pressure with two hands. I realize I may be in the minority. I just feel that if I can control pressure with one hand, I can control it with the other as well.
Also, I don't use the second hand to push down on the razor as much as to push it along the hone.
Again, I think its what works for you.
Jordan