Problem of user distinction solved, device distinction still a problem:
If I have two devices with hey Siri activated in the same area, both react... A possible solution would be that if two devices (with same iCloud account) get activated by the voice command, each one would give that information to the other devices before Siri reacts, and then determine the nearest by the voices level each device receives. Then only the nearest could respond.
Another option, instead of voice level detection could be to let Siri ask you on each device simultaneously which one was meant by asking for the device type (iPad, iPhone, etc...): "On what device do you want to ask me something?" - "iPad"
A last idea would be to let Siri ask first from the nearest device "did you mean me?" If users answers "yes", the user could go on with further commands on that device, or if he answers "no", the next device would ask the same question and so on ...
Just a thought, but maybe I am the only one with this "problem"