Capitalism works due to the "free market" which maps a large number of producers to a large number of consumers in an efficient way. When the market ceases to be sufficiently free, capitalism starts to falter. It may be regulation, it may be monopoly, it may be monopsony, it may be dysfunction of the judiciary system that fails to protect property rights, etc.

A system of only two parties trading the entire economic output of a country would be utterly un-capitalist, more like two kings trading in products of their kingdoms. No doubt that any social institutes would have been subjugated by these two parties.