An experiment in 2/1 twill on a rigid heddle loom, using two heddles at once. This effectively gives you a three-shaft loom, because the heddles can pull down as well as push up.
I'm including the threading diagram I created for this weave as one of the photographs. In summary, two threads pass through the slots of each heddle, and one through the hole. The threads that pass through the hole in the back heddle must pass through a slot in the front, and those that pass through a hole in the front heddle must pass through a slot in the back. That leaves 1/3 of the threads passing through a slot in both heddles.
You can use this threading plan to weave the twill with either the 2/1 or the 1/2 side facing you, depending on whether you leave one thread raised at a time or two.