Make sure you have enough “seed” in the audience you’re building a lookalike for in each market. Facebook’s guidance is at least 5,000. So if you have 5,000 “customers” in Canada, you could then build a Canadian seed that will perform with reasonable predictability.
Generally speaking, its best to create LLA by country. Performance between markets can differ wildly depending on time between Canada & the US, so create your separate LLA audiences by country, run them in separate ad sets and optimized based on results by country.
If you prefer to have CA & US together, you could still run a single ad group that includes both markets and has, for example, 2% Canada and 1% US — but keep in mind that regardless of what you do, if combining Canada & the US, Canada will only ever account for a fraction of your combined ad spend, since a 1% Canadian LLA is about one-tenth the size of a 1% US LLA.