While the disagreements between political parties have naturally always been present in the USA, their differences were historically nuanced, and Representatives of both parties regularly found common ground on a variety of issues. Party lines were not hardened back then, and movements like the Mugwumps (Republicans supporting a Democratic presidential candidate in 1884) were not unommon.
Starting in the 1970s, and especially in the the 2000s party lines thicken. With the election of Donald J. Trump, the divide of the USA has been more easy to see and deeper than ever.
(There is even somewhere on Stackexchange a graphic showing the likelihood of one representative to vote the same way as one from the other party. They diverge notably in the last 3 to 5 decades)
Let me be clear: This question is not grounded on Trumps victory alone. The political values of the Americans - be it universal health care and medicare, military spending, immigration policy, foreign policy, stance on the 2nd amendement, womens reproductive rights, social programs, and economic regulation - differ excessively; much more than they do in most countries.
In 2016 61% of the US population lived in so called "landslide" counties, the highest amount in recorded history. (https://fivethirtyeight.com/features/purple-america-has-all-but-disappeared/)
With a divide this deep, and world views so different, I ask this:
Would the complete seperation of the USA in two different unions not be advantageous for their respective population in reaching their political self-determination? Would such a scenario be sustainable? If not, how so?
DAA: ~170 million people
RUA: ~155 million people
One of the great advantages I see is that both can change their individual political system in ways that suits their philosophy, most likely radically altering their respective constitutions. Although easily mistaken that way from the picture, this doesn't mean each current american party gets its own country; but people of similiar ideology.
Once both countries were established, the "demixing" would spike for a short period, as people would move to the America of their values.