Keep in mind that trailer axles are built with a slight arch in them, i.e. The center is slightly higher than the ends. This is done so that when weight is applied the axle will deflect to being straight. If your axles are rated to high for your trailers weight then the axle will not straighten out enough and you will get wear on the outside edge of the tires. Consequently if your axle is under rated then it will arc in the opposite direction and cause wear on the inside edges. You can also have tire wear from bent spindles, wether vertical or horizontal. And finally if the axle is not perpendicular to the frame you will have wear issues as well. The point here is, if you are having tire wear issues then you really need to go to a shop specializing in trailer axle issues not an RV dealer.
PS. A Google search for double up axle comes up empty. I would ask them what they mean by double up. And although they are a specialty shop, if they don't have a good explanation I think that I'd get another opinion.
2015 3100RL legacy...2005 Ford F-250 CC SB. Tows like a charm! 4/19 Updated to 2017 Chevy 3500 CC SB SRW -hope it tows as well as my F 250 did!