I'm probably hampered by never having seen it done except in a couple of videos. The only treadling I've ever seen in person is a lathe which uses a different pivot point to put the majority of the motion at the front so it's done one legged with a pushing motion and knee flexion. For me it's completely natural to do it one footed - heel, toe, heel, toe, ..... No knee movement, just ankle flexion. I do intend to try to get the coordination of two feet into it at some point but each time I've tried so far it's been very herky-jerky.