Killing the two party duopoly requires significant constitutional changes to how elections are held. One way to do that would be ranked choice voting as implemented in e.g. Maine (https://en.wikipedia.org/wiki/Ranked-choice_voting_in_the_Un...).

However, the key point is that such a system would mainly benefit a potential third party, but any changes to the system need to be agreed upon by the current representatives whose interests and reelection would be hurt by such a change.