No,
you can define the base class and the joned subclass in different files... so:
FirstAssembly: contains the parent class and it's mapping.
Code:
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.0" auto-import="true">
<class
name="FirstAssembly.BaseClass, FirstAssembly"
table="BASE_CLASS">
</class>
</hibernate-mapping>
SecondAssembly: contains the joined-class and it-s mapping.
Code:
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.0" auto-import="true">
<joined-subclass
name="SecondAssembly.JoinedClass, SecondAssembly"
extends="FirstAssembly.BaseClass, FirstAssembly"
table="JOINED_CLASS">
</joined-subclass>
</hibernate-mapping>