I'd place at least the following conditions on this:
1. The governments of Canada and Quebec must eliminate their respective debts before Quebec can separate while sharing a common currency,
2. The Bank of... er... the Canadas?... would adopt a strict policy of currency stability, promoting neither inflation nor deflation, and would be politically independent, with its determination of inflation being based on the overall rate of inflation of both Canadas combined since that would be the Bank's jurisdiction, and
3. Neither government would be allowed to borrow without the consent of the other as long as they share a common currency.
Anything less, and I'D be wary of two nations sharing a common currency. Yes, sharing a common currency has its advantages in principle in that it eliminates the overhead cost of the money-trading middle-man, but we also don't want to trade that advantage in for some disadvantage. Some other agreement might be reasonable too, but where would you stand on this?