Still on twins OEI, when only one engine is pumping power big-time into the MGB, it is getting it asymmetrically, instead of one from each side. This puts stresses on the gear teeth, bearings, MGB supports, and the little people inside the box putting coal onto the fire.
The exception of course is the B212 / 412 where the engines feed to a combining box before it goes to the MGB by a single shaft.