UofT, you should try to submit your application as soon as possible. American schools have rolling admissions, meaning the sooner you apply, the better your chances. Even though you can't write the DAT until November, you should still submit your application as soon as you can. There have been reported instances (on SDN) of interview invitations even though DAT scores had yet to be submitted.
If you intend on applying only to American schools, then you could write the American DAT; however, if you intend on applying to Canadian schools, you should probably wait to write the Canadian DAT - Canadian schools won't accept the American DAT. As for diane's comment about schools not accepting the Canadian DAT, those schools also don't accept Canadians (or any non-US citizens).
The ADEA/AADSAS website might have a list of application deadlines. That's probably the best place to look.
I've never heard of anyone writing both DATs, but you may want to consider the time it will take you to study for the DAT, if you haven't yet begun. If it takes you 6 weeks to study, let's say, then you might as well wait the extra 2 weeks and just write the Canadian DAT instead of both.