The basis of a two-party system is actually single-member electorates. Because each seat can only have one representative and the economics of campaigning and organising make it hard for a third party to make appreciable inroads.
Australia's Parliament demonstrates the differences: the House of Representatives is composed of single-member electorates and is dominated by the two major political organisations (and this is for the good, as the government is formed in the House and a two-party system almost guarantees that a government can be formed and sustained easily after each election). The Senate, by contrast, is composed of multi-member electorates, one for each state, elected on a proportional basis. While the major parties typically have the largest share between them, the nature of a proportional multi-member electorate is that it creates more room for minor parties to obtain one or two seats.