This is a lot like data backups. Your data isn't really backed up unless it's on at least 2 different kinds of media, and also somewhere offsite. Using another

This is a lot like data backups. Your data isn't really backed up unless it's on at least 2 different kinds of media, and also somewhere offsite. Using another iPhone for the alarm is the equivalent of putting keeping your backups in the same server rack and having the data centre burn down.