I was trying to find an old article that I think was in Sound on Sound but a lot of the older stuff is not online. It covered some techniques for enhancing MIDI parts by creating duplicate tracks and then duplicating some or all of the notes.
I think a lot of what they suggested is now pretty mainstream like adding a sub bass note, like a simple sine, to some of the bass drum beats or to selected notes from the bass line. Mixed in so that it is felt rather than heard.
Another example was to take the right hand part of a piano and duplicate it to a marimba an octave or two higher, set the volume until you can just hear it then turn it down slightly. If available the suggestion was that you added some reverb to the marimba to create some high end gloss.
From memory a lot of the suggestions covered adding bottom end, mid range punch or high end gloss by adding some additional MIDI instrumentation based on an existing part pitched to fill the gap. And this was in the days when the processing and memory capabilities of MIDI sound sources was pretty limited. Not sure that these techniques made the sounds more realistic but executed with care they did make the overall sound better,
It may have been something like this or the layering of MIDI sounds that was being suggested.
Of course these days there are easier ways of achieving some of these effects.