Doesn't Cow have a nested ticket in his example, rather than a back to back? I've always understood a back to back is flying to where a plane either nightstops or does a turnaround and flies back to the origin, and it's a pair of tickets that covers both flights. Nested would be a second return journey within the end dates of another ticket.
It might take some fine reading of the Tariff, but I understood in Canada tickets need to be used in sequence. Even with nested tickets, you are using all the flight coupons in sqeuence
For Each PNR, but not if multiple PNR's are taken together.
Based on the Lufthansa case in Europe, for tickets there you might not need to use the coupons in sequence, although this is quite new. See this comment, plus the additional links to the case where LH sued a passenger for taking a nested trip, and lost.
https://liveandletsfly.boardingarea....egments-italy/
https://www.iftta.org/content/german...ross-ticketing