You don't get much of an increase in load capacity with a dualie. What you
do get is a little better stability from the wider rear track.

No one ton pickup is rated to carry 2 tons. That said, I have carried 3000
lbs of steel plate in my 3/4 ton Ram 2500 Cummins 4x4. The frame was just
hitting the overload springs. The net effect in driving it was it rode
more like a car than a truck. Loading was no problem because the load was
on pallets and forklifts were available at both ends.

It seems like GVWR ratings are chosen for liability reasons by the
manufacturers. Check out the ratings of the individual components on some
pickups and you will be surprised at what they will support.