There was
this thread and the end result was nobody could figure out if it was or not. Even the police did not seem to know. I read through the regulations, there is nothing specific in there I could find for RVs prohibiting double towing, lots about commercial trailers, that does not say I did not miss it.
That said I have only once ever seen it done in Southern Ontario and that truck had Quebec plates. My understanding had been it was only legal in Northern Ontario and only for boats behind a 5th wheel but I could find no confirmation for that more recent than like a decade ago so scratch that even if I have seen that done several times.