First of all, you probably wont get caught, however if the ADA does find out... kiss your chances of acceptance GOOD BYE...
American schools want American DAT scores, to take both the American and Canadian DATs then only send the higher score is highly unethical. If this practice became widespread it would completely undermine the validity of both tests.
EDIT: let me rephrase. If you are applying to schools in both America and Canada then SHOULD report the American scores to the American schools and Canadian scores to Canadian schools, at minimum. If you are JUST applying to Canadian schools, then I don't see a reason you would have to send them the American scores.