No- I mean hard code them rather than use the {category_menu} tags- so:
<p>Categories |<br />
<select name="category[]" size="4" multiple="multiple">
<option value='16'>Aug. 20-26</option>
<option value='17'>Aug. 27-30</option>
<option value='19'>June 12-18, 2005</option>
</select>
<p>Categories II<br />
<select name="category[]" size="4" multiple="multiple">
<option value='7'>Extensions</option>
<option value='6'>Modules</option>
<option value='5'>Plugins</option>
</select>
Replacing it with your options, of course. If it works hard coded, then you can use queries or something to dynamically populate each select- as needed. But try hard coding it first- you can rip the options off of the rendered code from the tag- just put the two groups each in their own select.
Won’t swear it will work, but can’t see why not.