Serialize alternate and neutral validating carrier in a separate XML element:

<AirItineraryPricingInfo>
....
  <TPA_Extensions>
    :
    <ValidatingCarrier SettlementMethod="BSP" NewVcxProcess="true">
      <Default>K0</Default>
      <Alternate>I0</Alternate>
      <Alternate>U0</Alternate>
    </ValidatingCarrier>
  </TPA_Extensions>
</AirItineraryPricingInfo>