alt Hacker NewsPlurality ("first past the post") voting systems always converge to a two-party system. There's even a theorem saying so, though of course the real world isn't always so polite as to behave the way the mathematics tells it to. Countries with better electoral systems don't so consistently seem to converge to a two-party system.
Caveat: it's per seat. Each seat gets two parties to pick from. There can be more than two parties in the government - some other force caused the US to agree on the same two parties in every seat.
When one party is secure in a seat and the second is weak, other parties can challenge each other for second place. For example libertarians could try to beat republicans in a place where republicans do poorly.