bassicly, you add two hidden form elements and two select lists:
<input name="on0" type="hidden" id="on0" value="color">
<input name="on1" type="hidden" id="on1" value="size">
<select name="os0" id="os0">
<option value="red">red</option>
<option value="yellow">yellow</option>
<option value="blue">blue</option>
</select>
<select name="os1" id="os1">
<option value="samll">small</option>
<option value="medium">medium</option>
<option value="large">large</option>
</select>
if this is not working for you, please post a link.