There are a "few" examples of airline within an airline working, like the aforementioned Air Canada/Rouge, Qantas/Jetstar. Others worth mentioning are Singapore/Scoot, IAG's Iberia/Vueling/Level (and the un-marketed IB Express), SWISS/Edelweiss (and the rest of the confusing constellation that is Lufthansa Group). The typical structure is a full service mainline carrier with a low cost, more leisure focused carrier. The problem tends to be when there is too much overlap in the carriers that they steal each others market share and become competitors. The market has to be large enough and segmented enough for both carriers to coexist. Finally, there is a cost in differentiating the carriers. Most often a mainline carrier will try to stave off a low cost entrant by starting their own "low cost subsidiary" with a cheaper labor contract - but then they lose focus on their primary carrier. In this example with WestJet - they can consolidate their market share as the spill carrier and AC alternative and avoid complicated labor agreements by consolidating under a single marketed carrier. Ultimately, even most "full service" carriers truly operate a hybrid model, as the true full service is typically only given on long-haul widebody service, subsidized by domestic and/or intraregional service with little differentiation between carriers.