How to ensure that at least one of 2 attributes is set to true

Hi List,

I'm new to XForms, read a lot but still missing something: I did not
find a solution (other than post-processing with xslt). I want to make
sure that at least one of the 2 attributes
(MIETE_PACHT, KAUF) is set to true (1). If this is not the case, I
would like to get a message or even prevent the document from being
stored.
Here is my instance and the bind:
<xf:model>
   <xf:instance xmlns="http://www.example.com">
   <immobilie>
      <objektkategorie>
         <vermarktungsart MIETE_PACHT="0" KAUF="0" />
      </objektkategorie>
   </immobilie>
   </xf:instance>

   <xf:bind nodeset="im:objektkategorie/im:vermarktungsart/@MIETE_PACHT"
type="xs:boolean"  />
   <xf:bind nodeset="im:objektkategorie/im:vermarktungsart/@KAUF"
type="xs:boolean"  />
   <xf:submission id="saveDoc" method="put">
      <xf:resource value="concat('test/fertigobjekt',
im:verwaltung_techn/ic:objektnr, '.xml')"/>
   </xf:submission>
</xf:model>

Input fields are:
<xf:group>
   <xf:input ref="im:objektkategorie/im:vermarktungsart/@KAUF">
      <xf:label>Verkauf: &nbsp;</xf:label>
   </xf:input>
   <xf:input ref="im:objektkategorie/im:vermarktungsart/@MIETE_PACHT">
      <xf:label>Vermietung (Miete, Pacht): &nbsp;</xf:label>
   </xf:input>
</xf:group>

Remarks about the example above and pointers to sources are also
appreciated. Thanks!

Manfred Staudinger