Originally Posted by LightingGuy
No, business as usual. I've had itineraries with both airlines on it. AA can't check me in for the AS legs, and with delays in the AA system had a short connection to the AS flights. Flights departed early, without me, since I "wasn't checked in and all checked in passengers were on board." I've since discovered this is much more likely if the AS flight(s) involve cross-border flights (like LAX-YVR).
I've had this same problem repeatedly with flights with multiple legs. Last week I did SAT-DFW-SEA-EAT. AA could check me in through SEA, and could check my bags all the way through to EAT, but could not print boarding passes for the SEA-EAT leg. Fortunately I had a long lay-over in SEA, but it would be a real hassle on a tight connection. AA has never been able to print the boarding pass for the third connection when it's on AS.