We’ve replaced a set of dually tires over several months by starting with the fronts, putting the best of the rest on the rear, and then buying two every other month, putting the newest in the front. Note that this is not the ideal way to go (replacing the whole set is a better idea) but if finances preclude that then it’s one way to do so.
With duallys it’s essential that tires be rotated IAW the factory recommendations for the truck. Because of the way roads are crowned there will always be uneven wear in the rear tires. Front tires wear more heavily as they do the steering and that increases wear. So keeping the set rotated is very important.
If you want more life then add the spare into the mix. This has several benefits, including having it removed regularly and keeping the storage/removal mechanism funtional. It’s not fun having to change a tire by side of the interstate in the rain (it’s invariably raining
) and finding out that the “retaining” mechanism is “frozen”. Or it’s flat, too. 
Duallys come with added costs and tires are one of those costs. They provide many benefits, however, and that’s why I (and, I presume others) pay the costs. 
G.