- Each of the two parties is able to reach an absolute majority in parliament
- When there are elections, usually one of the two parties wins an absolute majority
- The party that is elected wants to rule alone
- There's a large probability that with elections, the ruling party will change over time
The Median voter theorem describes such a system: While at first the parties have programmes that are very different from each other, over time, they will get closer. Hotelling's law also describes this in game theory.
Note that in such systems, there may also be other parties, bur these rarely have enough influence to win majorities in elections.