Well, it had to be done. Two Siris talking to each other.
But even at this early stage of Siri's development, all it takes is for someone to leave 2 Siris lying around and accidentally activate them and press their buttons at the right time in order to let them speak to each other.. and the consequences can be dire.
The outcome could get someone fired. The actual back and forth starts about 23 seconds into the video.
Update: Based on the antenna bands these are not 4S units. So, seems like it may have been staged/fake?