I don't think the US constitution was designed to enable two dominant parties; that's just the way things panned out.
10.1 History of American Political Parties – American Government and Politics in the Information Age (umn.edu)
The UK doesn't actually have a coalition government. The Conservative party has a majority in parliament, meaning they don't need any other parties' members' votes to pass legislation. Despite that, they needlessly threw the country into turmoil by holding the Brexit referendum. Largely because of the chaos created by Brexit, the Tories have been cycling through leaders* (PMs) at an unprecedented rate. So, holding that majority hasn't been a recipe for stability for them.
In contrast, Canada
does have a coalition government, of sorts. The Liberal party is in a minority position, meaning they have a plurality of seats in Parliament, but not a majority. To pass legislation, they need votes from other parties. For the past couple of terms in office, the New Democratic Party (NDP) has generally enabled the passing of legislation, as their combined seat numbers would form a majority. Up until a few years ago, the Conservatives (Tories) could actually be relied upon to add at least
some yea votes on bills. However, they have been steadily becoming the "Maple MAGA" party and under their current leader, Pierre Poilievre, that extreme partisanship has lead to them voting unanimously against legislation in knee-jerk fashion.
So, while the UK's majority government is a $hitshow, Canada's minority government is managing to get $hit done.**
My point is that coalitions don't necessarily lead to unstable governments, while one party in a majority position doesn't automatically lead to smooth sailing either.
*Unlike the US, countries with Westminster parliamentary systems of government, e.g. the UK, Canada, Australia and New Zealand, don't directly elect heads of government. Party leaders are elected by the party members and if their party wins a plurality of seats in an election, the party leader becomes Prime Minister.
**To be clear, I'm not a big fan of our PM - never have been - but I respect what he's managed to get done nevertheless. And, as long as the Tories insist on keeping their collective heads up their collective asses, I don't think I have any option but to support the Liberals.
