Form Dropdown Select Link

I have a select drop down, once user specifies the brand I want it to link over in a new tab to the page specified in the value. This works fine with 1 URL in the value but what if I wanted 2 tabs to open with 2 different URLs..any suggestions? Here is my current code.

     <fieldset>

      <legend><span class="number">1</span>Select a Brand</legend>

      <label for="Brand">Brand:</label>
      <select id="brandSelect" name="brand_select">
        <optgroup label="Grills">
          <option value="http://site1.com">DCS</option>
          <option value="http://site2.com">Alfresco</option>
          <option value="http://site3.com">Viking</option>
          <option value="http://site4.com">Fire Magic</option>
          <option value="http://site5.com">Lynx</option>
          <option value="http://site6.com">Coyote</option>
        </optgroup>
        <optgroup label="Outdoor Furniture">
          <option value="http://site7.com">Breezesta</option>
          <option value="http://site8.com">Polywood</option>
          <option value="http://site9.com">Malibu</option>
        </optgroup>
      </select>

    </fieldset>

    <button type="submit">Sign Up</button>
    <script type="text/javascript">
       var urlmenu = document.getElementById( 'brandSelect' );
       urlmenu.onchange = function() {
            window.open( this.options[ this.selectedIndex ].value );
       };
    </script>