Quick question I couldn't find the answer to...if Cersei Lannister and Cersei's Attendant are killed at the same time, such as through a two claim military challenge, or Wildfire Assault...do I get to trigger the attendant's ability twice? My gut tells me yes, since they are separate deaths, even though one source caused both deaths.

Yes. The death of each character is a separate trigger. You are Responding to the character dying, not the resolution of the kill effect.