B
For anybody else looking at fixing the issue with multiple simultaneous button presses it seems it can't be done.
Having dived into the M5Core2 source the events generated for two button presses, e.g. A & C, are press for A and move for A.
This looked like a deeper bug (since the second should have been identified as C), so going further in I found that an else branch wasn't refreshing the currently identified touched button on second press (C) in the method "doEvents" in touch.cpp (it doesn't help the code isn't documented, isn't single responsibility and has an arrow antipattern too).
Fixing that bug, so the correct button is refreshed, showed that two events together did not produce presses for A & C but instead (I assume - I'm fed up at this point) the hardware averages the presses' coordinates and declares it's button B(!) that is pressed.
If you have old Core 1 code you're porting that uses two simultaneous button presses I'd suggest changing the code to double taps of a single button instead which is the workaround I'm about to use - to remain consistent this behaviour is both on Core 1 and Core 2
More formally, I should have read the file header in touch.h earlier:
the M5Stack Core2 touch display is only
multi-touch in one dimension. What that means is that it can detect two
separate touches only if they occur on different vertical positions.
This has to do with the way the touch screen is wired, it's not
something that can be changed in software. So you will only ever see
two points if they do not occur side-by-side. Touches that do happen
side-by-side blend into one touch that is detected somewhere between
the actual touches.
