I'm no expert but I would say that using NAC at the same time as MMS would be counter productive. NAC induces the body to produce Glutathione, the master antioxidant. Antioxidants will negate ClO2 MMS, which is a selective Oxidizer. The ClO2 molecule doesn't stay in the blood for long so you could probably cycle between the two on a daily basis? Better ask someone more knowledgable then me on this one.
I have personal experience with taking Vitamin C at the same time as ClO2 and it just didn't work, at all.