Without meaning to sound like im stating the obvious, the point of having two engines is to make double the power of having one engine.
If one is 115 and one is 125 then yes one will be doing slightly more work than another, but then that is true of the cylinders inside your current engine, one will always make slightly more power than another no matter how carefully you try and build it (difference in intake characteristics and exhaust charactistics etc between the cylinders, not to mention thermal differences between inner and outer etc) but that doesnt stop them all being useful, and its the same with two engines, both will still accelerate the car.
If you have a big bloke and a very big bloke in tug of war on the same side, they both still do their bit.