Also, how did this Union come into existence?
it's hard to say, i definitely did it more for the aesthetics, the symetry around the st lawrence.
The main problem is that for this to happen Britain must lose upper canada, otherwise quebec/lower canada will just go with it. I think there are several possibilities, Spanish/French holding it wouldn't work, as they can't realistically control the great lakes from New Orleans, i think if you go back to the 17th century you could devise a surviving new netherlands that manages to take control of lake erie and the lower ontario, but it would be hard to explain the very convergent internal borders of this New England, hell, the concept of Vermont existing with a 17th century POD is laughable, on the other hand that is sufficiently far back that you can come up with an explanation as for why britain doesn't control the ruppert's land (powerful french navy? Danish presence?).
What i was thinking when i made this was a highly improbable situation where the US manages to get lower ontario during the revolutionary war, but in the decades following it there is another 1812 war like and New England secedes, now NE seceding IRL is already implausible, but them doing so with a weaker British presence in north america is even more. Not even talking about the fact that if britain could potentially just take back part or all of lower ontario in this case.
If the US controls the great lakes, then they control most of the rest of ontario and the prairies, although realistically i still think quebec would extend further along the ottawa river, maybe even to lake huron, i'm making a more serious take on a small canada where it is the case. As for the union lacking ruppert's land and labrador i think it would be because it's made an independant state, or international territory, or directly given to Newfoundland, or a sub-dominion that was merged with quebec but then left, i doubt the american could take control of it, and while the french had some control over the hudson bay as late as the 1780s that can't last without a base in the americas.
Now without an outlet for anglo settlers in lower ontario, many would stick around in quebec, the eastern township IRL were english majority until the 1880s, and montreal was english majority in the early 19th century, while not as many would go there as those who went to ontario IRL, i think they could make up several millions by today, and would historically have the best land, this would make the Quebeckers emigrate even more once they start lacking land, so more go to New England than IRL, in the end i can see this quebec having more people in total than IRL (maybe what? 10 millions?) but with less french speakers (say, 5 millions?). The large scale of their immigration as well as the fact that New England already has its own french speaking population and institutions for the acadians (who weren't deported, and their descendant make up 4 millions today, although half of that left the poor acadia for the more industrialized region of New England) means that the majority kept their language and stuck in communities, although i guess some would still be assimilated like IRL.
So this quebec and new england are separate dominion at first, but they would be close, and i guess the anglo-american upper class in the two dominions would be very linked. The economic difference between the industrious new england and the more agrarian quebec could slow down integration but maybe a common external threat (US?) could make an union, even a loose one, happen. The french speaking population would also be scattered on both sides of the border and there may be signficiant remittences sent from french new englanders to their quebecker families, there may even be an idea that by merging both dominion the french speakers could be stronger, so maybe they would be onboard too.
In the end it's still a rather implausible scenario, as i said i mostly did it for the look.