Some great answers here, but to try to give a "more marxist" view of the question ( also highly recommend Black Reconstruction in America by W.E.B. DuBois. Easily the best and most in-depth overview of the pre-civil war to post-reconstruction area I've read.) To pull a couple of quotes:

The South was fighting for the protection and expansion of its agrarian feudalism. For the sheer existence of slavery, there must be a continual supply of fertile land, cheaper slaves, and such political power as would give the slave status full legal recognition and protection, and annihilate the free Ne**o. The Louisiana Purchase had furnished slaves and land, but most of the land was in the Northwest. The foray into Mexico had opened an empire, but the availability of this land was partly spoiled by the loss of California to free labor. This suggested a proposed expansion of slavery toward Kansas, where it involved the South in competition with white labor: a competition which endangered the slave status, encouraged slave revolt, and increased the possibility of fugitive slaves. It was a war to determine how far industry in the United States should be carried on under a system where the capitalist owns not only the nation's raw material, not only the land, but also the laborer himself; or whether the laborer was going to maintain his personal freedom, and enforce it by growing political and economic independence based on widespread ownership of land

Quote 2:

The planters entirely misconceived the extent to which democracy was spreading in the North. They thought it meant that the laboring class was going to rule the North for labor's own economic interests. Even those who saw the seamy side of slavery were convinced of the Tightness of the system because they believed that there were seeds of disaster in the North against which slavery would be their protection; "indications that these are already beginning to be felt or anticipated by prophetic minds, they think they see in the demands for 'Land Limitation,' in the anti-rent troubles, in strikes of workmen, in the distress of emigrants at the eddies of their current, in diseased philanthropy, in radical democracy, and in the progress of socialistic ideas in general. 'The North,' say they, 'has progressed under the high pressure of unlimited competition; as the population grows denser, there will be terrific explosions, disaster, and ruin, while they will ride quietly and safely at the anchor of slavery.'"

Thus the planters of the South walked straight into the face of modern economic progress. The North had yielded to democracy, but only because democracy was curbed by a dictatorship of property and investment which left in the hands of the leaders of industry such economic power as insured their mastery and their profits. Less than this they knew perfectly well they could not yield, and more than this they would not. They remained masters of the economic destiny of America.

In the South, on the other hand, the planters walked in quite the opposite direction, excluding the poor whites from nearly every economic foothold with apparently no conception of the danger of these five million workers who, in time, overthrew the planters and utterly submerged them after the Civil War; and the South was equally determined to regard its four million slaves as a class of submerged workers and to this ideal they and their successors still cling.

TL:DR on this is that the Civil war was a fight between two different economic modes, the south was essentially still a feudalist, agriculture-based economy while the north was an industrial, capitalist economy. This came to a head in the American west, where the south essentially needed to expand to keep their economic system viable:

As the economic power of the planter waned, his political power became more and more indispensable to the maintenance of his income and profits. Holding his industrial system secure by this political domination, the planter turned to the more systematic exploitation of his black labor. One method called for more land and the other for more slaves. Both meant not only increased crops but increased political power. It was a temptation that swept greed, religion, military pride and dreams of empire to its defense. There were two possibilities. He might follow the old method of the early West Indian sugar plantations: work his slaves without regard to their physical condition, until they died of over-work or exposure, and then buy new ones. The difficulty of this, however, was that the price of slaves, since the attempt to abolish the slave trade, was gradually rising. This in the deep South led to a strong and gradually increasing demand for the reopening of the African slave trade, just as modern industry demands cheaper and cheaper coolie labor in Asia and half-slave labor in African mines.

The other possibility was to find continual increments of new, rich land upon which ordinary slave labor would bring adequate return. This land the South sought in the Southeast; then beyond the Mississippi in Louisiana and Texas, then in Mexico, and finally, it turned its face in two directions: toward the Northwestern territories of the United States and toward the West Indian islands and South America. The South was drawn toward the West by two motives: first the possibility that slavery in Kansas, Colorado, Utah and Nevada would be at least as profitable as in Missouri, and secondly to prevent the expansion of free labor there and its threat to slavery. This challenge was a counsel of despair in the face of modern industrial development and probably the radical South expected defeat in the West and hoped the consequent resentment among the slaveholders would set the South toward a great slave empire in the Caribbean. Jefferson Davis was ready to reopen the African slave trade to any future acquisition south of the Rio Grande.

This brought the South to war with the farmers and laborers in the North and West, who wanted free soil but did not want to compete with slave labor. The fugitive slave law of 1850 vastly extended Federal power so as to nullify state rights in the North. The Compromise of 1850 permitted the extension of slavery into the territories, and the Kansas-Nebraska Bill, 1854, deprived Congress of the right to prohibit slavery anywhere. This opened the entire West to slavery. War followed in Kansas. Slaveholders went boldly into Kansas, armed and organized:

"The invaders went in such force that the scattered and unorganized citizens could make no resistance and in many places they did not attempt to vote, seeing the polls surrounded by crowds of armed men who they knew came from Missouri to control the election and the leaders of the invaders kept their men under control, being anxious to prevent needless violence, as any serious outbreak would attract the attention of the country. In some districts the actual citizens protested against the election and petitioned the governor to set it aside and order another.

"We can tell the impertinent scoundrels of the Tribune that we will continue to lynch and hang, to tar and feather and drown every white-livered Abolitionist who dares to pollute our soil." 5 Shut out from the United States territories by the Free Soil movement, the' South determined upon secession with the distinct idea of eventually expanding into the Caribbean.

So yes, I would argue that they would be considered two separate nations in the Marxist sense (remember that in Marxism and the National Question Stalin describes a nation as "a historically constituted, stable community of people, formed on the basis of a common language, territory, economic life, and psychological make-up manifested in a common culture"). I would argue that without the shared economic life the US would not be considered a nation in the lead up to the civil war.

There is also the discussion about the post-civil war and the fact that black people in the south could be considered a distinct nation but this comment is already too long haha. I would recommend Black Bolshevik by Harry Haywood if you're interested in that side of things.