3 biggest problemsAs the thread says.
Bonus: have them form or be part of a larger EU-style trade bloc
It took over a hundred years for Canadian-American relations to be cordial. Up until World War I there was an expectation in America that Canada would become part of them. You’re asking for two very different nations along ethnic and religious lines to get to the same level of closeness achieved by two very similar countries in 150 years.
This seems the play. Otl america and Britain didnt start liking each other until Germany got grabby, which combined with the fact america couldn't beat the BE in a war of conquest until after the civil war (coinciding with the aforementioned germany issue) is what opened the door for amero-canadian relationsMaybe a common enemy? Have China invade Kashmir shortly after the Korean War. A coalition under the auspices of the UN, or the Commonwealth is formed to repel the Chinese. After the conflict the settlement resolves the competing claims of India and Pakistan. Continued Chinese belligerence leads to a NATO like security compact, led by the UK, later including Burma, Nepal and Sri Lanka. It expands a trade compact, and either prevents East Pakistan becoming Bangladesh, or makes the split more like Czechoslovakia. It’s a stretch, but I don’t see it as ASB.
What is the difference between Canada/US and India/Pakistan?
A European mind that's able to think outside the box.