For some people depending on their genetic snp mutations NAC can cause a bad reaction and is known to lower b12 in an effort to reduce excessive non liver reduced glutathione in the body which NAVE creates.
Fred and a few others mention horrible reactions, and others say it helps them. Have you gotten a 23andME test done? This would help to know where you would need certain supplements, dosages, what to take, and things to avoid.
I believe its mentioned that people with CBS mutations are the ones that cannot tolerate NAC, If you have low b12 levels I would also not suggest it as it will further deplete them.
It may be best to start the methyl folate first for a bit of time, watch for reactions and then add in the NA and see if that causes any issues for you. If you do it all at once you won't know what is going on.
Here is a good page on methyl folate and things to look out for, keep in mind you may get a detox reaction so feel lousy while the body does clean up work, and potentially have inflammation from dealing with said toxins.
http://mthfr.net/methylfolate-side-effects/2012/03/01/
You should be taking methyl b12 alongside the methyl folate otherwise it can deplete b12 levels and they are both dependent on each other for absorption anyways.
Another great addition would be Adenob12, Just be sure to raise your intake for potassium as start up of methylation from the methyl folate may significantly lower your intra cellular potassium levels.
At least an 3000mg a day is said to be the normal standard for proper replenishing.. The RDV is 4000mg.