> Enshrining the two party system in law is something that even the current power system in the US hasn't managed. By doing "top two to general" you'd be converting a de-facto duopoly into an actual duopoly.

Just the opposite, the top two candidates, regardless of what party they're from will appear on the ballot, this would allow people to vote for a 3rd party candidate in the primary without worrying about "wasting" their vote. If the 3rd party candidate is one of the top two they'll appear in the general, if not that voter can still pick the lesser of two evils in the general.