Speaking as a trucker , we dont change both tires , unless you have to travel a ways on the remaining good tire .. Many times if one blows , the other one isnt far behind .. If loaded near max weight ( 80000 # in most areas ) , there is quiet a bit more weight on the remaining tire .. So I could see the soundness of the changing both advice ..
My Suncruiser rear axle is rated at 11000# , the tires are rated around 3300# per tire ( another note , read your sidewall , you have two weight ratings , one for a single and one for a dual , the dual is usually about 10% give or take less than the single rating ) ..
On my tires if the rear is at max weight of 11000# and the weight is evenly distributed thats 2750# per tire at max pressure ( another side here , ratings are at max pressure , if you run less than max , then you're lowering the carrying capacity of the axle ) .. So if I blow a tire on the rear , I'm putting 5500# on one tire rated at 3300 , I'd be pulling over asap and calling road service ..