Complex sentence: A two-party system is a system where two major political parties dominate voting in nearly all elections at every level of government and, as a result, all or nearly all elected offices are members of one of the two major parties.
Split into simpler sentences: A two-party system is a system where two major political parties dominate voting in nearly all elections at every level of government. And, as a result, all, or nearly all, elected offices are members of one of the two major parties.