Towing Issues: dual wheels, wheel trucks, dual wheels

Question
Is there a difference in the towing capacity between dual and single wheel trucks of the same design? What are the advantages of dual wheels then?

Answer
in basic terms, each tire has a "rating" of what it can carry weight wise. It is marked on the sides of the tires in small print...

Now with a single wheel on each side if you have tires rated at 3000 pounds each then it can technically carry 6000 pounds TOTAL before you overload the tires.  

BUT that does not mean that the axle itself is able to carry that weight...

Dual wheels do tend to increase the capacity as you have 4 tires carrying the weight instead of 2. But again the axle capacity and all the related stuff is to be considered.

That is the easiest answer I can give without getting real technical.