In one-2-one chats, the sender is no longer assumed to be the other person.  The from address is read from the message.  Therefore if the current user is also chatting in another client, the correct username will appear on the incomming message.  That's the theory, but in practice, there's a bug in the hipchat XMPP implementation where it sets the 'from' address to be the other party, even for messages that the current user types in the hipchat client.