Tlk Bhai, you are confusing two thins, Munafiq and Atheist. Nifaq (Taqseem) within a religion does exist, has existed, and will exist because It does not deny the root of the religion. One can be Munafiq and still considered Muslim, but one can't be an atheist and a Muslim at the same time. One can not stay alive if head is chopped.
A cadaver is a cadaver, even if you put him in a cold storage, and hope that it will rise one day in this mortal world and before the day of judgement.
Atheist is not always a Murtid .. Athiest is the non-beliver who never develped that belief ..
Any atheist , at one given time .. could have parents who were a beliver of one faith or the other ..
if that faith (of those parents) was islam .. that means the athiest was raised in that family .. so one can call him Muslim athiest .. that would be just a label .. not a technical term . cause once you are athiest, you are not a muslim, anymore