<schema xmlns=“www.ascc.net/xml/schematron” >

<pattern name="Attributes present">
     <rule context="BBB">
          <assert test="not(@aaa) or (@aaa and @bbb)">The element must not have an isolated aaa attribute</assert>
          <assert test="not(@bbb) or (@aaa and @bbb)">The element must not have an isolated bbb attribute</assert>
     </rule>
</pattern>

</schema>