Amend the xsite configuration and the xsd

<global>
  ...
  <sites local="LON"/>
  ...
</global>

<global>
  ...
  <site name="LON"/>
  ...
</global>

     <backups backupSites="NYC,SFO">
       <backup site="NYC" backupFailurePolicy="WARN" strategy="SYNC" timeout="12000"/>
       <backup site="SFO" backupFailurePolicy="IGNORE" strategy="ASYNC"/>
       <backup site="LON" strategy="SYNC"/>
     </backups>

     <backups>
       <backup site="NYC" failurePolicy="WARN" strategy="SYNC" timeout="12000" enabled="true"/>
       <backup site="SFO" failurePolicy="IGNORE" strategy="ASYNC" enabled="true"/>
       <backup site="LON" strategy="SYNC" enabled="false"/>
     </backups>