All my treadle wheels are single except for the Ashford Traveller. My husband bought that one for me. He followed the advice of Susan (Susan's Fiber Shop).
When I started using it, I only used one treadle, then played with using both treadles.
I now use the Traveller more than the wheels with a single treadle. I do find (now) I have more control with it.
I think the bigger question is one drive band (Scotch tension) or two!!
That's why I love my Traveller, it can be either a single or double band drive