Obviously a one way connecting flight of just over $250 is possible on the AC website. Why pay for two flights at a steeper rate?
There is nothing wrong with doing the two separate tickets other than it will cost you more.
Remember there are other connecting points from YUL (eg. YYC-YUL is a direct flight too). Try pricing to YYC and YYC-YVR but I still think it will be more than an all in one ticket using the more advanced options on the AC website.