You have a very good question and it is not so simple because scientists so far do not know the exact answer to that. The current thinking, as Jim quoted, is that both sporadic ANs and NF2 (bilateral ANs) have NF2 mutations. Nf2's usually now have genetic testing but sporadic ANs are not routinely genetically tested, so it is impossible for scientists to say what the differences are. The idea that both types of ANs arise from the same gene stems from studies of some sporadic ANs which have been genetically analyzed by scientists. The studies showed that sporadic ANs have NF2 mutations.
Nowadays the diagnosis is given based on WHERE these mutations are. If the mutations are found in the blood, it is a full-blown NF2 case because presumably every single cell in the body is mutated. If there is only one AN and no other scwannomas, meningiomas or gliomas anywhere else and no NF2 mutations are found in the blood, then AN is called sporadic, unless the person is very young (below 30) when the case is suspect. In this case only the tumor cells are mutated. More often that not, AN is assumed to be sporadic unless proven otherwise. People who develop one AN, other tumors but have no detectable blood NF2 mutations, are called NF2 somatic mosaics. They have mutations in some cells in the body besides the tumor but not all and it is impossible to say which ones.
The fact that NF2 is inherited is a little bit of a misnomer because about 50% are spontaneous mutations, that is, they have no family history of the disease but have it themselves and can pass it on to the offspring.
NF2s develop ANs at a younger age, within a few years of each other. Their ANs are more aggressive and more difficult to remove. There are plenty exceptions to this "rule", even on this board. There are NF2s who developed their ANs within 20 years of each other, relatively late in life, and so on. Moreover, just because somebody is known to be an NF2 does not in any way predict the course of their disease. Nobody knows how many tumors if any the person will develop during the course of his/her life.
I do not know if it is possible according to the current thinking to have an AN if the person is diagnosed with NF1 because NF1 involves an entirely different gene. NF1 gene has been isolated earlier than NF2 gene and before that nobody distinguished between two types of Neurofibromatosis. It is possible to have been diagnosed 20 years ago with NF1 only to be reclassified later as NF2 based on the presence of an AN.
I hope I helped and did not confuse you any further.