It's effectively a feature, but the amount of intentionality is grey, I think bourgeois democracy initially fell into this happy outcome, and it's a natural balance that occurs from them acting in self-interest, but it is also maintained actively with laws, culture wars, etc.

Which party people vote for seems to entirely depend on "the other guy is worse", so if both parties do terribly for people when they get in government (and they will because they serve the bourgeois class) then people will always vote for the "only other choice" and voteshame third partyers.

Rinse and repeat and the balance will stay forever around the 50% point while conditions worsen and people remain under the illusion they have a choice.