There isn’t a two party system. It is only bipartisan because such a system generally reduces to two in a polarized and popularity system. even though through state or province they will often pick a 3rd party, sometimes those people will decide that their 3rd party doesn’t have a chance they just do not want to see an opposing party win so they’ll vote via popularity vote instead of their preferred 3rd or 4th party. They settle. And I’m not sure a rating system would reduce the chance of that happening either.