I had never noticed before but since I just washed my hands, it seems I do it every time. I pour (or rub a bar) into my left hand, rub it with my right, swoop over the top of both, go back to my palms, in between my fingers, cups with both, splash water on top of the other, rub again together and air dry. There ya go!! I do wash the top of my left and right hands more than once a day
I don't use the back of my left hand. I use the front of my right hand. However, there does not seem to be any way to wash just one hand, so ... I dunno, just erratic intervals.