There may never be an issue with hierarchy, dogs often live happily together without any probs, other then the odd squabble over food. personally I have always found bitches to be more stroppier- When they are play fighting they may already be passing on signs that you are not reading - i.e one dog may always be below the other, rolling over and looking up at the other dog - . sorry if i have not expalined this well
regards
DT