Something

<tr>
       <td>
          PLZx / Ort
       </td>
       <td colspan="2">
          {$form.address.$blockId.postal_code.html}
          {$form.address.$blockId.city.html}
          {$form.address.$blockId.country_id.html}
<script language="javascript">
  $("#address_{$blockId}_postal_code").autocomplete("modules/foebud_civicrm/scripts/plz.php",
  {literal}
  {
                selectFirst: true,
                extraParams: {ort: function() {
                  {/literal}return $("#address_{$blockId}_city").val();{literal}
                } },
                formatItem: function(row) {return row[0]+" "+row[1];},
                max: 50,
                minChars: 2,
                width: 300
        }).result(function(event, data, formatted) {
                {/literal}$("#address_{$blockId}_city").val(data[1]);{literal}
        // }).focus(function() {$(this).flushCache();
        }).width("5em");
  {/literal}
  $("#address_{$blockId}_city").autocomplete("modules/foebud_civicrm/scripts/ort.php",
  {literal}
  {
                extraParams: {plz: function() {
                   {/literal}return $("#address_{$blockId}_postal_code").val();{literal}
                } },
                formatItem: function(row) {return row[1]+" "+row[0];},
                minChars: 3,
                cacheLength: 1,
                selectFirst: true
        }).result(function(event, data, formatted) {
                {/literal}$("#address_{$blockId}_postal_code").val(data[1]);{literal}
        // }).focus(function() {$(this).flushCache();
        }) ;
{/literal}
</script>
       </td>
</tr>