| Chess Pwn |
they don't stack and you use the best/any that apply.
flurries don't interact at all, you would have two separate full attack actions and they'd behave separately.
The AC bonus just applies as normal If you have a +1 from monk and a +1 from brawler and the conditions for both are met, aka you're a lv8 character 4 in brawler and 4 in monk and you're wearing no armor then you get you'd have a +2 to your AC.
Their unarmed strike damage is tracked separately and you'd use the highest available. AKA at lv6 a monk 3 brawler 3 would still only do 1d6 with their punches.
Why are you wanting to combine them? They really don't work well together as a multiclass.