Even before America existed as a country it was known that slavery would be a divisive issue. The founding fathers "solved" this by simply not talking about it. They left really solving the problem to future generations.
When Jefferson first drafted the Declaration of Independence it contained anti-slavery sections. These sections were dropped but what if, at this time, a different conversation took place. Let's say the founding fathers thought the slavery issue would have to be addressed sooner rather than later. Someone suggests that two USAs could be created. The idea is tossed around for a bit (publicly) but ultimately rejected. In order to defeat England, the colonies must stay unified. But the wording of the debate changes a bit: The colonies must stay unified to defeat England, but it is not necessary they stay unified after victory is achieved.
The "Two Americas" solution is part of the national debate for a decade. Lots of smart people refine it, but a "One America" is generally deemed much better. But when the Constitution is getting hammered out things come to a head. The North/South differences are deemed intractable. Two separate political entities are formed. The United States of Southern America has Virginia on down. The United States of Northern America has everything else. These are "Brother Nations": they have separate constitutions and federal governments, but they agree to fully support each other for their mutual defense, and they agree that neither federal government can impose tariffs on the other nation.
Is such a thing totally ASB, or is there some slim possibility it could have happened.
If it did happen, what results?