I believe the stacking has to do with lethal/nonlethal damage. Brawl give you 1d6+Str nonlethal; while CMA gives you 1d4+Str lethal or nonlethal, your choice.
As I rule it (may not be how WotC rules it), Brawl is an unarmed attack. CMA makes your unarmed attacks armed, and therefore, not subject to AoO. Therefore, Brawl is not subject to AoO if you also have CMA.
Likewise, CMA is an unarmed attack. Brawl grants a +1 bonus to hit for unarmed attacks. Therefore, CMA attacks get a +1 to hit if you also have Brawl.