No problems with two F1 races in one country. The USA is big. The Austin stop will be called the "United States Grand Prix", but at the end of the day, that's just a title with no entailment of exclusivity.
I wonder what the NJ race will be called?
added: See
http://en.wikipedia.org/wiki/United_States_Grand_Prix. Lots of examples of multiple F1 races in one year in the US. Only one in any given year is called the "United States Grand Prix". Austin gets to be called that only because it was organized first.