I htink I replyed to a similar post a while ago, but I'll take it again.
All you need to do is change the selectbox_id.selectedIndex property to another number.
If you want the same row on each box to be active:
<select id="selectbox_1" onChange="selectbox_2.selectedIndex=this.selectedI ndex">
and the same thing is done for the second box if you want them to change eachother.
Otherwise you can do like this:
<select id="selectbox_1" onChange="selectbox_2.selectedIndex=this.options[this.selectedIndex].value">
to make the second box change to a different option depending on what the value is of the option you just selected is.