It doesn't appear that it's possible to search 2 LDAP server for sponsor lookups by modifying the ajax.args.server field. As far as I can make out the ajax.args.server field only supports one server.
A workaround for this is to add 2 x sponsor_lookup fields to your guest registration page.
As you can't add the same field twice you'll need to duplicate the sponsor_lookup field then add it again: