What if Canada had just been lower Canada and the Maritimes, and Upper Canada had ended up American following the American Revolution?
How do you define Upper Canada? The entire Great Lakes Littoral, or just the region surrounded by lakes Ontario/Erie/Huron.
The Maritimes will look to Britain, while Lower Canada won't. Of course, Lower Canada is clearly the dominant partner. I imagine this will result in great amounts of tension, and if/when the US stops being seen as a would-be conqueror by the British, this union will likely break.
IOTL, a border which was seriously discussed was the Nipissing Line, which looks like:
View attachment 385690
Upper Canada didn't exist, of course. I imagine such a border would result in Western Canada falling under American rule as well, though a bit of "Upper Canada" would remain British.
Maybe some different divisions of states before the Civil War to appease the South, Also I guess the Americans also get all of Western Canada because they have a good choke in upper Canada