Well, there've been pictures of this and even instructions on how to cause it in the chinese general discussion section of the forum. One way is to get lucky with the random hero selection, another involves making use of lag...
Without going into the details of how the trigger works, let's just say that two people clicking on the Mystia/Sakuya at roughly the same time (depends on lag, much harder to do on LAN) will be able to both select Mystia/Sakuya at the same time.