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).
As a fan of drumming, you're probably one of the millions who have seen this. But for anybody who hasn't, here's a fun video showing how complicated drumming can be with a live performance by someone whose nickname is "The Octopus".
While the entire video is fun, the link is cued up to a place where there's a lot going on, but it's slow enough that you can kind of see what he's doing. Watch his knees to see what he's doing with pedals. Notice that his hands seem to be going in cycles, but there are subtle variations in the cycles.
At 8:54, strobe lights start up - that can't make it simpler to keep this focus. What he's doing looks essentially impossible to me, but I'm well aware that I don't know enough to understand just how insanely difficult it really is.
https://youtu.be/FssULNGSZIA?t=465
Nice. Oh yeah for sure that makes sense. I also think of the locking in a limb to play a preset "pattern loop" is also really similar to juggling which is something I enjoy. Just a lot of overlap in general!