The argument for 2 aircraft in Devon and Cornwall will be the geographical size of the areas and the relatively poor road links compared to many other UK areas.
Add in the fact that the population doubles when the tourists arrive - which seems most of the year when Covid doesn't prevent travel - and the need to get to the Scillies for Cornwall and you start to see why two is a better option.
A central charity could still promote itself locally just as they do at the moment so the idea that the public are donating to their own AA would still be there.