I started off using one hand, but discovered that despite my notion that my left hand would be unusable (as I am right handed), I am actually MORE effective using two hands. When shaving the left side of my face, the razor seems to fall naturally at the right angles when held in the left hand, as opposed to my right. It was a challenge getting my hand to go where I wanted at first, so I just took it even slower. It is a bit more natural now.
As with most things in straight razor shaving, my $.02 is that there really is no right or wrong, only what gets you results. Give the other hand a shot though, you might be pleasantly surprised at how well you can actually use it.