You have to give a hint to Dozer about implementation of Base. Otherwise it will try to instantiate interface and that is not possible.
<mapping>
<class-a>Wrapper</class-a>
<class-b>Wrapper</class-b>
<field>
<a>bases</a>
<b>bases</b>
<a-hint>BaseOne,BaseTwo</a-hint>
<b-hint>BaseOne,BaseTwo</b-hint>
</field>
</mapping>
http://dozer.sourceforge.net/documentation/baseattributes.html
alternative (update):
<mapping>
<class-a>BaseOne</class-a>
<class-b>BaseOne</class-b>
</mapping>
<mapping>
<class-a>BaseTwo</class-a>
<class-b>BaseTwo</class-b>
</mapping>