Both of your sentences indicate that the two companies are Number 1 and Number 2 in the whole of North America, and that there no companies from either country ahead of them. Neither sentence is the equivalent of saying that Company A is top of the US and Company B is top in Canada, and you could stand accused of making an unfounded claim.
If you need to indicate that their status is country-specific, you do need to say that.
What you could say (and probably justify) is that the two companies are among the most successful in the United States and Canada; but again, that doesn't indicate that either is top in their respective country.
How about something like the following (which I've just noticed you have suggested in the question title)?
We refer to Company A in the United States and Company B in Canada, the most successful in their respective countries.