While I'm sure the bureaucrats who came up with that had the best of intentions, and perhaps it is still true with some companies, its not really the practical answer for most people.

Every insurance company I've ever had has issued one card showing proof of insurance, and that card has been valid for both the US and Canada. I'm pretty sure I've even shown such a card when asked by a Canadian border guard without any problems at all.

By all means, a quick phone call to the insurance agent is a good idea before traveling to Canada, and they may have a different card, but I'd say at least 90% of the time, as long as you make sure you bring your regular proof of insurance card, you'll be just fine.