I've done that and had it work, what you need to do is assign a String [] to your second select box (the one you're trying to get the data from) and replace the submit button with a standard html:button with an onclick event. The onclick event should iterate through the select box entries in javascript and set them all to selected then do your forms[0].submit.click();
Struts doesn't update the array used in your html

ptions tags so if it was null before you'll still get null when trying to access the data afterwards. You might like to use an event handler to catch the enter key press and ignore it on that page too so the hidden submit button it only accessable by you through your javascript.
Hope this helps, let me know if you didn't understand any of it