The back pressure of the large engine would work against the efficiency
of the smaller generator and when only one of them was running you are
putting a back pressure on the other that can do all sorts of bad things
to engines and even humans.


Not mentioned but perhaps implied; The running engine can force the standing
water in the idle engine exhaust sytem back up to the engine exhaust elbow
and possibly into an open exhaust valve.

Steve
s/v Good Intentions