Xml Schema - value types from another part of xml file

Question

Hello, I'm developing an xml schema for xml that looks sth like this:

<sync name="Zlecenia" requires="Klienci" direction="OneWayToSource">
      <source>
         <table name="vwCustomer">
             <field name="FirstName" />
             <field name="LastName" />
             <field name="Name" />
             <field name="Company" />
         </table>
      </source>
      <destination>
         <table name="soCustomer">
            <field name="Name" mapsTo="FirstName" />
         </table>
      </destination>
</sync>

In the destination part I have tables with fields. Some fields are going to have parameter mapsTo set. I would like the values of that parameter to come from field names from tables in the source part. Is something like that at all possible with xml-schema?