You could have put someone on both on stage and make him interact with duplex. Of course it could also have been staged but more difficult. By picking a succes

You could have put someone on both on stage and make him interact with duplex. Of course it could also have been staged but more difficult. By picking a successful recorded try you tend to obfuscate failure rate. As opposite Apple often demonstrate feature on stage with real call to outside people or live Siri command. And as with real tech it sometimes fail. They always have a backup hardware and they sometimes use it if the first try is a fail.