I showed Peter Jekins my self-advancing system in 1998 that is still better than the Sherrills rig by one point.

You need to have your friction hitch/device positioned at a point "higher up" the rope so your puling down stroke is with your best power.

By adding a lenght of webbing loop runner on your center harness attachment and then attaching your climbing rope/device/split-tail place it where you can still reach the hitch/device to descend.

This added distance will still let the hitch/device self-advance but now with a double foot loop on the down rope you can use both hands and both feet to ascend.

See you at the top,
Dan