It won't work
Simply put, the way the U.S. is set up now means that any long-term attempt at semi-amicable situation (or even one where there is animosity and tension but not outright war as with India and Pakistan) is doomed to failure.
I'm going to try to outline a question I submitted to Politics SE about could the U.S. successfully split, but since it got deleted without even notifying me and I can't find it anymore I won't have the whole argument.
The primary dividing lines that the U.S. would probably split along are between the liberal-leaning West and East coast and the conservative/libertarian-leaning interior. These zones are easily separated by the Cascade and Appalachian mountain ranges to the east and west, making them readily defendable. Florida could go either way. Alaska might try to form its own independent government, though in the long term it would more likely be conquered and colonized by the West coast. Hawaii would have to do something, it relies too heavily on imports for its standard of living, and couldn't maintain that on its own.
The biggest, biggest issue is that the interior and the coasts both have critical resources that the other needs for long-term survival. "Flyover country" contains most of the United States' arable land, as well as most of its strategic resources such as iron, natural gas, strategic oil reserves, and uranium. However, the coasts have near unilateral control over trade, and without access to the coasts or the St. Lawrence seaway the interior will become highly dependent on trade through the Mississippi, and the interior's economy will crash without easy access to the sea, especially in states like Illinois, Ohio, Idaho, and Montana. Lack of sea access is typically devastating for a country's economy (look at Bolivia).
In terms of arable land and crops, California and Florida do produce a huge amount of the U.S.' food but overall the highly populated coasts rely on the interior to support them. There just isn't enough arable land otherwise (which would be made worse if the parts of Washington and Oregon east of the Cascades use this as an excuse to finally leave like they've been grumbling about). However, when it comes to arable land, most of the U.S.' winter food is grown in California and Florida, which means the interior is likely to run into food shortages and famines once winter hits.
In the case of California, a much bigger issue is water. The major cities of California like Los Angeles and San Diego are dependent on the Colorado River for water (both for drinking and agriculture) and hydroelectric power. Control over the Colorado River is a big deal, so much so that California and Arizona nearly went to open war over access to it, with Arizona setting up machine gun nests along its side of the Colorado in preparation for open war. If the U.S. split the primary dams controlling the flow of the water would end up in the interior, in a position that would be really hard for the coast to maintain military control of due to being on the other side of Death Valley. If California didn't have access to the Colorado River its population would die of thirst and it would no longer be able to be an economic breadbasket.
Another issue becomes the question of "who gets the silverware". Washington D.C. is east of the Appalachian mountains, so it makes sense that the East Coast would get D.C., but who gets control of America's military hardware and the numerous military bases dotted around the country? Do they remain in the original government's hands, or get split 50-50, or geographically? Do some of these new nations have foreign military bases dotting their soil, creating positions of vulnerability? Who gets the nukes? Only two of the 10 known nuclear silos are outside of traditional conservative strongholds (mostly in interior states), but those two silos hold 2/3's of the United States' nuclear arsenal.
This doesn't get into external geopolitics. China would try to seize control of the U.S. as best it could, trying to convert it into a vassal state if they saw weakness. The U.S. is still useful as a beaten, shackled rival buying Chinese goods to keep their economy afloat. Japan, South Korea, Australia, Europe, and Israel would panic at their largest supporter against external enemies vanishing. They would probably end up throwing military support behind whatever faction wished to maintain a single United States regardless of the citizens' wishes, creating an El Cid-style scarecrow for protection. This would result in whichever side wanted to reunify the States through force seeing a huge amount of overseas funding and materiel support.
Canada might fall apart with the prairie provinces (Alberta, Saskatchewan, and Manitoba, maybe with B.C. joining in) wanting to join the U.S. Interior. Canada has similar problems to the U.S., in that the prairie provinces produce the bulk of Canada's food and has most of its fossil fuels, but have almost no say in how these resources are used because Canada has a straight majority system based on population rather than an electoral college with two houses and so the Lawrencian Consensus (Ontario and the parts of Quebec along the seaway) almost totally dominate Canadian politics. Canada has a lot of the same problems that the U.S. does.
Militarily, the bulk of the U.S. population is in the coastal states (though some of these are in provinces west of the Appalachians), as well as the largest cities, so the coasts would have more troops to draw on. The interior would have much lower population densities, but gun culture means you would have a lot more people familiar with fighting and thus easier to train as higher-quality soldiers. People in the U.S. often like to joke that if such a split as described would happen and led to civil war between the liberal and conservative parts of the U.S., it would be over in five minutes because most left-leaning Americans are soft city folk who don't even know how to fire a gun, completely ignoring the fact that you can train someone to be battle hardened very quickly and our own history (i.e., the Civil War) shows that large numbers of undertrained troops can easily overwhelm more highly trained troops and better commanders. This is oversimplifying things, but one of the big reasons the Union won the Civil War was it was more industrialized and more populous, and therefore was willing to play "who can get the higher casualty rate" with the Confederacy after a while.
The other question is whether the United States would be willing to peacefully separate in the first place, and that one would not seek to simply conquer and subjugate the other for ideological reasons rather than logistics. This is something that is actually talked about in the case of eastern Washington and Oregon wanting to secede and join Idaho because they are so politically disenfranchized by the Sea-Tac area and Portland having near-unilateral political power. People on the coasts do not want to allow eastern Washington and Oregon to be allowed to leave and self-determinate, because they claim it will be a breeding ground for "white supremacy" or backwards ideals. Current left-leaning politics in the U.S. is becoming increasingly authoritarian, the rhetoric of the coasts increasingly being "you are too stupid to know what's good for you and we must force our ideology on you whether you want to or not, even if you just want to be left alone". On the other hand, if the interior does secede and adopt more conservative, religious policies, you could easily see some new-age firebrand pastor trying to incite a war against the coasts to reclaim "lost land". So it's either a crusade in the name of woke-ism or a crusade in the name of backwards old-school religious values. Neither sound like good options.
This isn't even getting into the fact that most separations don't end peacefully. Especially because in this case the primary lines of disagreement are not between geographic regions, but between neighbors. Look at what happened with the Partition of India. Or, to look at American history, a good example might be the state of Missouri, where the most brutal fighting of the Civil War happened. Missouri was split in terms of its loyalties, and the confederates in particular engaged in brutal terrorist actions where they hunted down, tortured, and murdered pro-Union civilians in horrific manners, including women and children (indeed, in some cases they deliberately targeted children), and then blended back into the civilian population. In response Union General Ewing responded to the guerilla warfare and terror tactics with the time honored tactic of killing everybody. He told the population they had 15 days to get to places of safety within military outposts, and then has his troops kill every man, woman, and child that he found outside these safe zones (whether they were Union supporters or not) and burned their houses, there are even records of troops killing entire families execution-style in front of their homes, including the elderly. It worked. This is what you would get if you tried to divide the United States today.
So, what would probably end up happening is that you would separate amicably for a few years before all out warfare would erupt over control of the Colorado River at the Hoover Dam (yes, really. No, I'm not talking about Fallout: New Vegas). This is a do-or-die scenario: California and the coasts' breadbasket will starve without Colorado River water and hydroelectric power, whereas the cities of the Southwest and the Colorado Plateau will die without access to fresh water. This would result in open warfare, and eventually one country would conquer the other. You would have a United States again, but with one country turned into a conquered vassal nation with its inhabitants turned into second-class citizens.
And this is honestly just the tip of the iceberg with regards to how issues in U.S. geopolitics would turn secession into a nightmare that would probably end through violent subjugation and conquest at best.