May be u can try this:
Code:
<!-- ... -->
<class name="Message" table="Message">
<!-- id go here -->
<many-to-one name="sender" class="Person" column="Sender_Id" />
<many-to-one name="receiver" class="Person" column="Receiver_Id" />
<!-- other properties -->
</class>
<class name="Person" table="Person">
<!-- id go here -->
<set name="sentMessages">
<key column="Sender_Id" />
<one-to-many class="Message" />
</set>
<set name="receivedMessages">
<key column="Receiver_Id" />
<one-to-many class="Message" />
</set>
</class>
Book: "Hibernate in Action" may also help u a lot.