<fileset basedir="${source.path}">
  <exclude name="/**"/>
  <include name="a*.lib"/>
  <include name="b*.lib"/>
</fileset>

<fileset basedir="${source.path.parent}">
  <exclude name="${source.path}/**"/>
  <include name="${source.path}/a*.lib"/>
  <include name="${source.path}/b*.lib"/>
</fileset>