That's another point where the meaning of the Constitution is debatable, I believe. When I read it, it seems obvious to me that it means "at least" two parties. Both the parties in power want to interpret it as "only" two parties, of course, since they are one of the two. I'd also like to see a "none of the above" choice on the ballots, though that probably won't ever happen, either, because then there would be no way to hide the fact that most voters would not pick either of the two parties' candidates.