I have not started using a Straight for my dome yet, but doing the back by feel is just second nature. You make your ass, and run your finger tips along the pass to determine where the edge of the previous pass is, and repeat. The advantage of "feel" shaving is that it allows for you to use your free hand to stretch the skin, to whatever degree possible, while making your passes.
Besides, a second mirror just seems like it would complicate things in terms of the perspective (left is right? or is left still left?).