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).

Depending on the city pairs you were looking at and if you have A list status, it might be possible to book 2 flights on the same day for different city pairs that don't overlap timewise, with one flight being later than your ideal time. If you end up needing the city pair on the later flight, you can use the standby benefit to fly earlier (and cancel the other city pair). I've never tested this though and don't know that anyone has reported that this works.