I've come across this situation a few times and always ruled that no they do not stack with each other. But there always seems to be one person who says, "but brass knuckles make brawl deal lethal damage."
The feats are in the D20 Modern core rulebook.
BRAWL- when making an unarmed attack, you recieve a +1 competence bonus on your attack rolls, and you deal nonlethal damage equal to 1d6 + your STR modifier non-lethal damage
IMPROVED BRAWL- +2 competence bonus, 1d8 + STR modifier non-lethal
KNOCKOUT PUNCH- non-lethal strike unarmed is automatically a critical hit
IMPROVED KNOCKOUT PUNCH- non-lethal unarmed citical hit deals X3 damage
STREETFIGHTING- +1d4 damage once per round with unarmed attack or light melee weapon
COMBAT MARTIAL ARTS- with an unarmed attack you deal lethal or nonlethal damage[your choice] equal to 1d4 + your STR modifier. your unamred attacks count as armed.
IMPROVED COMBAT MARTIAL ARTS- unamred strike threatens critical hit on 19 or 20
ADVANCED COMBAT MARTIAL ARTS- unarmed strike critical hit deals X3 damage
BRASS KNUCKLES- when used by a character with the brawl feat, brass knuckles increase the base damage dealt by an unarmed strike by +1 and turn the damage into lethal damage.