It' s two separate airlines and from the OP' s information two separate tickets so this isn't cabotage as it' s not being sold as a through trip.. When you land at Trudeau you go upstairs through a long corridor towards CBSA; there are stairs midway leading to US departures and US CBP. You may or may be allowed to go that way. They will definitely not check bags all the way through. You'll have to claim them and check them again with AC. This is not a connection, but rather a trip to Montreal followed by a trip to another city from Montreal. It' s important to keep in mind that if your first flight is delayed and you miss the second one you will forfeit the second ticket if it' s non-refundable, and your first airline is not responsible to re-route you if the flight is delayed, only to get you to Montreal. Good luck!