Do creatures with the same damage reduction such as 2 balors ignore each others DR? I thought in 3.5 they did, but i cant find it in pathfinder rules.
As far as I know, no they do not automatically ignore each others dr. They would need a weapon that is both cold iron and good aligned to bypass a balors dr.
"Some monsters are vulnerable to magic weapons... Such creatures' natural weapons (but not their attacks with weapons) are treated as magic weapons for the purpose of overcoming damage reduction."
and
"A creature with an alignment subtype (chaotic, evil, good, or lawful) can overcome this type of damage reduction with its natural weapons and weapons it wields as if the weapons or natural weapons had an alignment (or alignments) that matched the subtype(s) of the creature."
Their DR applies unless stated otherwise or are using something that states it ignores it.