Aside from OTL, a number of ATL scenarios have explored different north/south splits of the continent between various nations. "Dominion of Southern America" is the one that sticks out most in my mind. But these have still usually created a split along the north/south.
I'm interested to see what possible PODs could lead to an east/west split, which would result in at least two independent, stable nations on the continent by 1900. The border could be based in part on a major river (e.g. Mississippi) and would need to extend far enough north into OTL Canada and south into OTL Mexico (or the Gulf) to effectively bisect the continent. Ideally neither nation should have seaports in both the Pacific or Atlantic/Gulf.
Bonus points for three nations split this way!
Ideas?