The issue would be that you would need to have crews trained for a situation that might happen once a year? Plus the maintenance of the platforms, snow clearing, etc. It would just make sense to Split the number of trains that use the track.My suggestion years ago was for VIA to create some switchover tracks where the CN/CP lines meet at various points from Toronto to Kingston.
Then, if there was a freight delay on the CN line, the train could negotiate to use the CP line to bypass the stalled or slow freight train.
However it would require a lot of coordination and probably some kind of advanced center to coordinate things.
CP is also hesitant to allow VIA and other passenger service on their lines, but maybe they would be more forgiving if it was just seen as a backup to the CN service.
Plus that wouldn't work between Belleville and Smith Falls, since you would bi-pass Kingston and Brockville.