Interestingly, it seems airlines (at least the relatively major ones) tend to have interline agreements with pretty much every other (relatively major) airline that participates in codesharing agreements and sells flights via GDS systems, even if they're not "partners"--in other words, everyone has agreements with everyone else EXCEPT LCCs like WN that don't interline with anyone and don't sell flights on GDS systems.

So, even though AS and UA are polar opposites when it comes to who they are partnered and allied with, you can still interline your bags and even buy a multi-carrier ticket (generally through a travel agent) involving both airlines.