Can we separate this issue from Aeroplan and Star Alliance tickets? Tickets involving different (non same alliance) airlines are pretty common with those issued by travel agencies. Basically they are contract/wholesale rates with options on different airlines if the major airline cannot get you to the final destination.
For example - before codesharing was widespread AC ticket between YYZ-SIN will involve AC to HKG or NRT, then another airline of choice between HKG/NRT - SIN..... CX, SQ, NH, UA etc etc...
I think you can still find them on ITA Fare Matrix by selecting the Multiple Airlines option