I am guessing the one lock nut guys assume the tension between the 2 boxes will maintain the intimate contact necessary. I guess they do not understand the (thermal) linear expansion thing.
Actually I was surprised that the "two locknut" language was only in the over 250v article. I have heard it so often I thought it was any time you used RMC/IMC directly into a box through a KO. Usually when I saw threaded steel pipe it is going into a hub tho.
