True, but he's got to get through his own province first, to get to a state in order for the state laws to apply :)
I don't think OPP will be convinced if his excuse is, "But officer, I was headed for Michigan, and it's perfectly legal to double-tow there!"
It's not like here in NY where it's illegal but the fuzz doesn't seem to give a hoot. From what I've heard, double towing is not a moving violation and they have to have another reason to pull you over before they can ticket you for double towing...
BTW, that website says "Not Stated" with regards to "triple (i.e. double) tow."