Sociopaths "mimic" the behaviors of society to gain trust of an individual. For another psychopath or sociopath, that wouldn't work - because things simply have no emotional value to them (from my understanding).

Psychopaths - don't neccessarily even try to form a relationship of any kind (even as far as "friendship"), so this would be almost inapplicable to the psychopath, but for those that do get in relationships - they would only be attracted to someone they felt they could manipulate or control, which would not fit the description of another psychopath or sociopath so psychopaths would not be in a relationship with another psycho/socio-path either (in my opinion).