Form Dropdown Select Link

I have a select drop down, once user specifies the brand I want it to link over in 2 new tabs for the 2 URLs. How can I do this, is it best to do onclick event for submit and then run a function with "if and else" through the selects..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", "http://site1A.com">DCS</option>
          <option value="http://site2.com", "http://site2A.com">Alfresco</option>
          <option value="http://site3.com", "http://site3A.com">Viking</option>
          <option value="http://site4.com", "http://site4A.com">Fire Magic</option>
          <option value="http://site5.com", "http://site5A.com">Lynx</option>
          <option value="http://site6.com", "http://site6A.com">Coyote</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>