A friend of mine is redoing his 1990 Dodge 4x4 Half ton.

He is putting a flat bed on it. Lives on a farm,

So Now he is bugging me on how he can put dual rear wheels on it??

I have heard of this, but really have no idea.

Any one ??

You could, buy why?
It's as easy as swapping in the dually axle with the same gear ratio and running the same size tires front and back. Then you may be able to swap in a HD yoke on the transfer case to match the HD driveshaft. Or, custom make a shaft. Most if not all 3/4 & 1-tons have a suspension lift on the rear that you may not want.
All this leaves a half ton suspension and frame to pile stuff on. Sounds like it's easier to get a 1-ton farm truck if that's what you really need.

JC Whitney used to sell a kit to do it

Your friend is also severely overloading the small axle bearing and shaft. Just because there is dual wheels the weight hauling or GVW does not increase. Waste of money in my opinion.

Your friend is showing his ignorance.

Dual wheels usually have less traction than single wheels in slippery situations. I learned that trying to drive an empty grain truck through the snow.

As said above, the tires are not the limiting factor in load capacity.

If he really "must" have dual wheels the best option would be a complete rear out of a 3/4 ton dually pickup.

R.