Well, the stories say that Dobermans can be same sex aggressive. Two males, two females, there is a chance that when the second dog reaches sexual maturity (or both dogs, if you're making the equally dubious choice to raise them together), there is a chance that the dogs will become aggressive to members of the same sex and want to fight.
This is problematic, as we do not typically want dogs to fight. Not unless you're a scumbag.
