its likely that the effects of one are potentiating the effects of the other. The bigger issue is a persons response to the drug and how their body metabolizes it. I am not a physician just some with a keen interest in pharmacology. CNS is your central nervous system, this system control much more than just breathing, certain drugs may affect the heart more while others affect the breathing more. the issue becomes troubling when the cns is suppressed enough that one of sub systems (breathing or heart) becomes so low that it either stop functioning and arrest begins or in some cases it may be that they are both still "functioning" but not properly and hypoxia can set in. My understanding with GHB and alcohol is that its the heart rate dropping which causes the dangerous effect.