Good thing we have XML Schema elves:

  <xs:element name="pizza">
     <xs:complexType>
        <xs:sequence>
           <xs:choice>
              <xs:element ref="my:toppings"/>
              <xs:sequence>
                 <xs:element name="half" form="qualified" minOccurs="2" maxOccurs="2">
                    <xs:complexType>
                       <xs:sequence>
                          <xs:element ref="my:toppings"/>
                       </xs:sequence>
                    </xs:complexType>
                 </xs:element>
              </xs:sequence>
           </xs:choice>
        </xs:sequence>
     </xs:complexType>
  </xs:element>
  
  <xs:element name="toppings">
     <xs:complexType>
        <xs:sequence>
           <xs:element name="kind" form="qualified" maxOccurs="unbounded" type="xs:string"/>
        </xs:sequence>
     </xs:complexType>
  </xs:element>