I hope this helps explain how I have combined the same four shaft overshot threading onto eight shafts. And you can see why you need 12 treadles. Using just 16 ends as an example become 32 ends.
You use the same weft for both pattern and tabby, alternating from those on shaft 1-4 plus tabby on shafts 11 or 12 and then on the next pick will be a different color to weave pattern from those on shaft 5-8 plus tabby from shaft 5 or 6.
When you alternate your colors pick-by-pick and then switch that order when you start a new repeat, you get two different-colored motifs that appear side by side.