First, I'm just curious, what do you want to haul that is 6' wide and over 14,000 lbs or so?
Now, the forum probably won't give me enough space to list all the advantages of a dual tandem, so I'll just hit the high spots.
You have 8 tires hauling the load, not 6.
If you have a flat, the tire beside it will let you limp a ways. W/ a single, it usually completely destroys the tire, and often the rim, before you can stop.
Three axles make the trailer much more resistant to turning, so much so that if you don't have enough weight on the drive axle, in a slick spot the truck will turn, but the trailer will go straight.
Sharp turns put much more stress on the trailer, since 3 axles don't pivot like 2, but instead you have to drag one sideways.
Tire wear is usually much worse on a 3 axle.