Re: Bach, also factor in that an organ player’s two hands would often be playing on two separate manuals (keyboards) that produced different sounds from each other and from the footboard. The player could activate a different set of stops (groups of pipes that provide a consistent sound for each note playable in the range of the keyboards) for each keyboard or footboard, and could also vary those selections to some extent during a piece’s performance.

If you look past conventional terminology, an organ really a single instrument in a similar way as a multi-input and multi-output synthesizer is a single instrument today — the difference is more that even a modern organ generally encodes most of the technology limitations of Bach’s time, whereas a computer-based synthesizer’s additional sound flexibility reflects the broader technology capabilities of our current times.

For that matter, a kit drummer is essentially playing (up to) four instruments at once since each limb could be doing something unique (granted for many (most?) drummers, they’re just using the high-hat pedal as a footrest, beating quarter or half notes on the bass drum and their hands will be focused on a single instrument, but a truly talented drummer might have four unique rhythms going at once). Add in the fact that some drummers sing backup and it can be quite impressive.

I had the pleasure of working with an amazing organist in my time at Holy Name Cathedral (he’s apparently since become semi-retired, just doing occasional fill-in work). Watching him do a Messiaen organ piece as a postlude after Mass it was like seeing video of Keith Moon on the drums. Limbs moving in a seemingly chaotic manner and yet music came out the other end (on top of crazy multi-manual keyboarding and pedalwork, he was also frequently changing the stops on the organ as he played).