I know that several manufacturers offered them as an option back in the 50's and 60's but I have to wonder about the reasoning behind it.
I can see the need for them if you are forced to mow grass cross-slope because they change the center of gravity. But are there other applications where duals would make life better? After all, you have to be conscious of those tires sticking out the sides of the tractor, all the time. From a mechanical aspect, duals put a lot of strain on the axle shaft/flange joint which is usually just welded together.
And once you start adding heavy counterweights to offset front mounted attachments, the strain is increased even more.