Says a lot about the world that the flight deck drew outnumbered the cabin-talk about productivity.
As to the two radio ops aside from the fact that away from land it was all HF radio much of it was W/T wireless telegraphy. And back then while pilots could understand Morse code I am not sure they could send it . Also the Radio Ops would be required to keep a listening watch so having a pilot spell on operator means he isnt getting his rest as apilot . The radio sets were nothing like today's digital frequency selection , they required a lot of experienced and deft fingers to tune the frequency precisely and know when certain frequencies work and when they dont. HF is a very funny medium in terms of propagation you could often hear a station thousands of miles away and not hear one 200 miles away . Keeping a listening watch with all the background noise on HF is mentally very tiring as well and I am sure the whole flight crew were pretty darnn tired at the end of a transatlantic trip at 300 odd knots on a very small flight deck with four great piston props rumbling and vibrating away