While this used to be possible, the auto cancellation of conflicting reservations started both with the no-overbooking policy and the new IT system used for flights starting spring 2017
From what I can tell, and from what others have reported here, it will auto cancel if you have 2 reservations with overlapping flight times, or if you have multiple flights in the same direction between city pairs in the same day (e.g. 2 MDW-BWI flights on the same day at different times).
Currently have a SJC-LAX flight arriving into LAX at 12:50PM. Looking to book SJC-BUR departing at 1:45PM or 3:10PM for the same date as I am unsure if I will be able to make the LAX flight. Will WN still detect this as double booking and cancel one of the reservations?