How to do multiple column UniqueConstraint in hbm?

Use the properties tag:

...
<properties name="uk1" unique="true">
        <property name="username" .../>
        <many-to-one name="client" .../>
</properties>

<properties name="uk2" unique="true">
        <property name="email" .../>
        <many-to-one name="client" update="false" insert="false" .../>
</properties>
...

Documentation extract :

The <properties> element allows the definition of a named, logical grouping of the properties of a class. The most important use of the construct is that it allows a combination of properties to be the target of a property-ref. It is also a convenient way to define a multi-column unique constraint.

All available options are described in the Hibernate documentation.

You can also do this:

  <many-to-one name="client" unique-key="uk1,uk2" .../>
  <property name="username" unique-key="uk1"  .../>
  <property name="email" unique-key="uk2"  .../>

You do not need to use the tag in hbm . If you only want multiple unique constraints.