LecLightning56 wrote on Mar 31^st, 2019 at 8:00am:

Also, it would appear from the original Microsoft ESP for the sound configuration files that only 4 engines are permitted in FSX, hence each 'engine' has to represent the paired engines on each of the four pylons.

And that is the answer to why each pair of engines is controlled as if they were a single engine. Captain Sim did explain this somewhere in this forum a long time ago.

I'm not sure about the engine sounds, but I'm guessing you sorted it out when you say "The result is awesome!". ;)