The above example is not a back-to-back, however, it is a "nested ticket" which is also subject to some rules. Basically, the philosophy is that you cannot overlap 2 discounted tickets. However, as others have stated YOW and YYZ are far enough apart, that you should be able to book all segments on one ticket and still receive the discounted, Saturday night stayover fares.

Regards,

Ken Hamer