[dude with glasses in a communist t-shirt, arguing] I'm the only leftist here, your opinions are TRASH
[dude holding a theory book on smug, arguing] Read theory you losers, you're all WRONG
[dude in an anarchist hoodie, arguing] Nuh-uh, I'm the only leftist here, you're SHITLIBS
[the three dudes are now caught in a cartoon fight, glasses gone flying, punches everywhere, while a firing squad of nazis are targeting them with rifles]
[a confused nazi asks] Why… why are they still arguing?
Americans informally create coalitions. That's why you hear the term "caucus" a lot more often, like Bernie Sanders "caucusing" with Democrats. Many libertarians may not like Trump and the fascist Republicans, but they still caucus together. The problem with caucusing with Democratic party is that they sideline the left, especially Bernie Sanders, in favour of more corporate friendly candidates. As for the Republican party, well the right always act right and value group cohesion and appeasing the rich more, even if they become fascist.
Caucusing is hardly working and here is the hard to swallow pill for Americans: organise grassroots campaigns and plant actual progressives into primaries. Americans used to be good at doing that. That's how they got the Roosevelts, ended the first Gilded Age, and third party candidates being elected more. The duopoly system became entrenched sometime after the early 1900's, probably when Theodore Roosevelt ran third party and split the vote of progressives, which handed the presidency to the racist Woodrow Wilson.
Caucausing isn't really comparable to coalitions in my opinion, because all the formalisms are missing.
Bernie Sanders has no actual power within the party, no matter how many people voted Democrats because of him.
Compare the situation to an actual multi-party system with coalitions. Sanders would have his own party and there would be 1-3 other parties that are currently part of the Democratic party. Each of these parties would collect separate vote shares which would lead to some of these parties being larger and others smaller. Voters would have to choice to express which exact political direction they prefer instead of just having a binary choice.
After the election, coalitions would be formed. These coalitions wouldn't have to be along the current party lines, but e.g. moderate republicans and moderate democrats could form a coalition with eachother. This way, coalition-based multi-party systems tend towards moderate compromises, while two-party systems tend towards extremism.
In a multi-party system centrists represent reason and compromise, whereas in a two-party system they represent boring blandness.
In a coalition, each of the coalition partners hold power, because everyone of them can end the coalition. This means, more compromise is necessary and someone like Sanders cannot just be ignored for decades.