Is it possible for my father to be a primary driver one 2 cars in Ontario?

It's very possible, even in Ontario. But it's also very possible for the insurance company to see right through the bull shift. You're talking about literally the oldest trick in the book.

Remember this, and don't forget it: Insurance companies don't ask questions they can't look up the real answers to, no exceptions. When they ask how many convictions you've had in the last three years, they'll verify your answers against your provincial driving record. When they ask if you've been in any accidents, it's because they can look that up too. And when they ask how many vehicles and drivers are in the household (and they will, twice), it's because they can look that up too. And it you lie to them, you'll get caught.

Even if you slip one past their front lines and get a policy issued, you're still not out of the woods. There's a clause in every policy known as the misrepresentation clause, and they use it. The clause says that if the named insured (your mom in this case) commits any misrepresentation (lying) or non-disclosure (lying by hiding the truth), the whole deal is off. The contract is declared null and void, and they don't have to provide any coverage. And the truth always comes out after an accident, always.

You've probably heard a few horror stories, where someone bought a car and insured it, made all their payments on time, and then an accident happened and the insurance company used some technical fine-print reason to weasel out of paying the claim. Those stories are usually true, and the legal technicality is the misrepresentation clause. It happens all the time.

Yes, a person could own 3 or 4 cars, and be the only person that drove any of them.

BUT...

If you state your Mother is the main driver when she in fact ISN'T, the insurance company will see that as fraud. They see people trying to run this sort of scam all the time to try and get cheaper rates.