By encouraging discussions from both ends of the political spectrum, perhaps? The discussions themselves cannot be neutral, but the environment can be.
events of r/askreddit offer possible insight, some months ago someone posted a question similar to "Trump voters that will not vote for Trump again, what was the last drop?" and it became a huge post, soon after the same question was posted again by only changing Trump to Biden (in true r/askreddit fashion where popular questions are milked to death). Only one of them was removed by moderators.
A political discussion forum can be neutral in allowing equal participation to different political position. People are not asking reddit userbase to be neutral in their voting, they are asking reddit moderators to be neutral in how they enforce policies/rules.
