I know of no official protocol for sending to more than one specific device at a time - throughout all of computerdom.
Both devices could hear - but only one - whose address is packaged with the information - would accept the information - and ask for corrections.
One at at a time. Error correction - as well as security concerns - are among the technical reasons for this. Doesn't mean I can't listen to ALL the noise out there being sent and received - it just means that error-correcting and secure protocols aren't designed to "broadcast" more than short beacons like "I'm here!".
Good luck!