OK, we were in exactly the same situation - I was coming from California, we were married and he "spoke for me" at immigration to get me documented status to stay with him while awaiting finalization of the PR ap. We drove into Canada together in my car and immigration never raised an eyebrow - however, someone at Customs, as we were walking out, told us that if my Canadian husband was caught driving my car in Ontario without me in it, it would be impounded. Obviously they didn't realize I had added him to my California auto insurance policy - and, in fact, there were two occasions when he was "caught" driving it without me present and it wasn't an issue. The first was as a result of his ex trying to make trouble for us, and the second when when he was rear-ended on the freeway and the car was totaled.
That said - trying to bring two vehicles into Canada, at least at the same time, will be problematic. You will likely not be allowed by Canadian customs/immigration to drive a vehicle into the country that is not registered in your name. She can be with you in her car, but you can't bring one of her cars in by yourself. So the only option, that I see, for bringing a second vehicle in is to leave one States-side and then fly back (or take a bus/cab) in a few weeks and drive that one in, too. But remember, each time she leaves Canada, she'll have to go through an entrance interview again and there's always the chance that someone will get pi$$y and refuse her entry.
Bottom line,
she can't import either vehicle until she has permanent residence. If she transfers ownership to you, you will have to pay duty, whereas, as a landing immigrant, she doesn't have to pay duties on either vehicle. If you can, it would probably be a lot easier to bring one up for her use and leave the other in the States until she has permanent status. Also,
exporting and then
importing vehicles is a bit time consuming and complicated - so you might want to review that information, too, before you even decide for sure that you want to import either car.