[rgl]

Или даже так:

<script type="text/javascript">
  (function() {
    var select2 = document.getElementsByTagName('select')[1];
    var optionStore = [];
    for( var i = 0; i < select2.options.length; ++i )
      optionStore[i] = select2.options[i];

    var select1 = document.getElementsByTagName('select')[0];
    select1.addEventListener('change', function() {
      var i = this.selectedIndex

      if( select2.options.length >= 12-i )
        select2.options.length = 12-i;
      else
        for (var k=select2.options.length; k<12-i; k++)
          select2.options[k] = optionStore[k];
    });
  } )();
</script>