dual main engine a single output

Nope, it is not! It boils down to the thrust load per disc area and forward speed, or, if you use the classical Taylor propeller diagram, it is a function of the load factor "Bp" (=nprop*Hp^0,5/Va^2,5).

This means that with a Power split and clever selection of propeller shaft speed and diameter, you may obtain equal (or better) efficiency with twins than with the single prop. All depends on what factor constitutes the limiting dimension.

 

Is that when prop tip speed gets too high so you need twins?
but the single engine will beat the 2 at its best rpm ( BSFC) but outside that the flexibility of the 2 would most likely be better
So that might also mean driving 2 props with one engine could be better as well.