There are a good reasons to play in a 'both back' formation when returning in doubles, one of them being to counter the big server and the vulnerability of the returner's partner against the server's partner when in the 'up' position. This is demonstrated well here by Pablo Carreno Busta and Feliciano Lopez last week at Wimbledon.
See also Lopez movement into the split step and his reduced backswing on the FH return to counter to speed of the oncoming speed. It was a good serve into his body and whilst he managed to hit a winning return down the line the net player probably should have done a better job with his technical anticipation (ie: Lopez was never going to find the inside of the ball here to go back cross-court) to cover it.
Carreno Busta is not passive in his role - see his movement on and just after the return, ready for the short reply.