The reason for the two alternate realities in the same country is primarily to do with what information you consume.
As an exercise, go to the front page of Reddit everyday. You will almost always notice on the first page or the first few there are always posts on politics. And the vast majority of these posts are the "bad" takes/actions/scandals on rebuplicans/conservatives.
If you go to r/conservative , you will see the "bad" takes/actions/scandals of the democrats/liberals.
On Reddit the echo chamber for democrats is the entirety of Reddit except for a select few subs. The echo chamber for conservatives is in their own subreddits. On conservative websites this is the opposite.
If one looks only at the Reddit front page, no doubt they would not like the republicans because it's a highlight of their low points. I.e a Reverse Instagram, post all the bad parts of your life.
This is the issue. The two realities is from getting information about how "good" your side is and a feed about all the bad stuff from the other side.
The reason why you think 50% of the population is like that is precisely because the information you have received about that side is their bad highlight reel. And they will equally think the other 50% of the aisle is brainwashed because of all they are viewing your bad highlight reel