At present, my Singer 328K sits in my Singer straight-leg treadle. But before purchasing the 328K, I had a National Two Spools sitting in my Singer straight-leg treadle.
I had my first Two Spools in a modified treadle that my husband did the modification for me (clever man!).
Since the Two Spools machine is narrower, he took a wider portion of another Singer cabinet (lighter color wood shown) to hold the machine on the front lip.
All I do is unscrew the hinges from the wooden front piece and switch the wooden pieces that fits with the correct machine. The older light wood which is wider goes with the National Two Spools and the newer narrower wood goes with the Singer machines.
Both Singer pins can't be use to hold the Two Spools because the pins holes on the Two Spools don't line up with the Singer hinge pins.
Since only one pin will line up for the Two Spools and can only be used, my husband used a metal piece for support for the second pin. The machine rests on the hinge pin and metal plate.
This metal plate can be moved out when using the Two Spools and swing back under cabinet for the Singer Machines.
Location and how the metal plate is attached to cabinet.
I hope this helps others in their one treadle, two heads projects! Have fun treadling!