Apparently Apple has a bunch of heuristics to determine which Siri to activate when there are multiple iDevices listening (and a non-zero amount of the world population is trying to figure out what they are). I really hope they make "Hey iPhone Siri, stop the alarm" just work so that I don't have to yell at a confused iPad twice every morning. Support for "Hey Siri #31174" or "Hey Lord Sirious" would have been nice too.