This quote is what I found on Woodland Woolworks about the double/single treadle difference:
"A double treadle wheel balances the treadling work on both feet and legs and may therefore be more comfortable and ease strain on the hips and lower back, especially over long periods of spinning.
With smaller wheel diameters, especially in upright wheel designs, lower wheel momentum may make treadling feel jerky and make it difficult to maintain consistent rotation especially at slow spinning speeds. Double treadle mechanisms overcome this by providing two "power strokes" per wheel revolution. Starting the wheel is also easier."
Hope that helps a little