I can see one potential advantage to having two valves, if one fails the other will at least keep the back end somewhat level. Of course if the valve valve fails in the release air position that would defeat the purpose.
Like Dan said, I've got no clue why two are used. Our 310 only has one and it's mounted near the center of the rig.
Air forums # 1674
1974 20' Argosy Motor Home
1974 31' Excella trailer (parting out, as of 4/1/2015 I have wheels, brake drums, windows & holding tanks left to sell)