Compared to a truck with singlewide tires in 2wd, how is the traction compared to a dually in 2wd(onroad)?

My experience is that the dually turns much more stable on road curves at or above the posted speed.

Turning is more stable. If you think about it, you will have better traction. You have more surface area contacting the pavement.

If you're talking like drag racing, i've seen a 2500 be able to anialate the rear tires and I have to do a boosted launch just to make 'em squeal.
So yes, I hook up better.
The flip side to that is in the rain, it's incredibly stable, if you start messing around with it the extra weight of the second wheel makes one HECK of a sliding turn.