In reality, there are more than 2 parties in the US. For instance, the third biggest party is Libertarian. The issue is that in order to be admitted to tv coverage and debates each presidential candidate has to show more than 15% of recognition (which is conducted through surveys). And the situation is that only 2 parties are usually capable of showing these numbers. For example, Ross Perot was the third candidate during 1992 and 1996 election and he represented Reform Party. Or there was a businessman George Wallace who run as independent in 1968.