AA owns 3 of the American Eagle carrier's: Envoy, Piedmont, and PSA. Delta owns Endeavor.Some majors used to have regional partners (maybe AA still does?) as wholly-owned subsidiaries. Still had separate labor agreements, separate workforces, separate operations teams, etc.
Horizon Air is a subsidiary of Alaska, but you cant take a flight attendant from one and staff the others plane.
Heck, up until this week, a United flight attendant that came from Continental couldnt work a plane that came from pre-merger United (or vice versa). So, being a subsidiary wouldnt help any.
As for mainline flight attendants, legacy AA and legacy USAir only now (1 October), have achieved Flight Attendant Operational Integration into one system, and can bid on any flight or aircraft they are qualified on.