see here:
https://rpg.stackexchange.com/...efit-from-two-armors
that, of course, is not an official source but it shows how stacking can work and the sort of stacking I have in mind when I say yes you can stack and yes things have changed.
It is absolutely correct that AC, or armor bonus to ac, does not stack. you get the highest of all contributors and that's that. However, Things like Quilted cloths DR is not an armor bonus.
Note how they say about where the normal ac is bypassed that the force armors armor bonus works. this shows a different reason for amror stacking then just for more ac, you can stack for 'back up' armor already on you.
In piecemeal armor, Linked by Cornelius and the paizo link is below.
https://www.d20pfsrd.com/gamem...les/piecemeal-armor/
There are mechanics for how different layers of armor will change your armor into a heavier tier when it has like layering as heavier armors. This can also be used in the reverse. Changing the way AC stacking works, you actually have a way to wear mithral plate over a chain shirt and its half plate body armor. Like buying pieces of a suit rather than the whole suit. They are effectively stacking but mechanically counting as a higher armor. Then, say something is sundered, you lose that piece but otherwise only downgrade the armor mechanics. This also works with magic armor, you no longer need to toss or sell that mithral chain shirt youve ran with for 10 levels.... just because you are ready for some full plate. You can piece it with other armor to keep the magic and have the new armor bonus.
That example I showed, was pushing it in regards to light armor, its weight is some 20 lbs and its quite a few layers. It would probably actually count as a type of medium armor.
People have also dissected the act of putting a +1 haramaki with + effects in with a +5 platemail... and presented you get the +5 platemail and the effects from the haramaki. I'm trying to find that forum thread, when I find it I will post it.
The piecemeal mechanic creates segement zones of armor, you can actually have just a sleave be enchanted, or one enchantment on one sleave and a different enchantment on another sleave, while the rest of your armor is mundane.
(these rules also account for partially wearing armor, cool stuff).
(also, as a note, in there is the Lemallar cuirass and the wording is shifted enough to clarify RAI. I just saw it.
*Lamellar Cuirass: This cuirass is made of lacquered leather plates bound together and fitted over a silk shirt. It is sometimes worn alone, especially during ceremonial occasions when wearing other armor is frowned upon.
Demonstrating you are right, in large, on this cornelius. two out of three so far. Unfortunate, seems me and several others were wrong about it and I will have to find another decent base.)
My class is winding down now so I will be posting in the IC thread soon.