There are at least three different McNitt/McNett families in the U.S. and no connections between them have been proven:
1. The McNitts of Palmer, Massachusetts - descendants of Barnard McNitt, many of whom have adopted the McNett spelling.They quickly spread across the country.
2. The McNitts of Mifflin County, Pennsylvania - Most of the early generations of this family stayed put in Pennsylvania rather than moving west, so they are not as spread out geographically as the Massachusetts McNitts.
3. The McNetts of Virginia and West Virginia - Unlike the other two families they are not of Scotch-irish origin.The surname Maquinet was gradually corrupted into McNett after the family settled in a heavily Scotch-Irish area of Virginia.They have since spread to many other states and are sometimes confused with other McNetts who descend from Barnard McNitt.
In general you can guess by geographic origins.If you find a McNitt or McNett that you can trace back to Massachusetts or New York, they are probably descended from Barnard McNitt.If they trace back to Pennsylvania (especially Mifflin County), then they probably descend from the Alexander McNitt who founded this family.If they trace back to Virginia, they are probably Maquinets.There are exceptions to these guidelines, but they work pretty often.