Я у себя это решил так
[CODE]<input id="arrFilter_pf[CITY][RIGHT]" type="hidden" value="" name="arrFilter_pf[CITY][RIGHT]"/>
<select onchange="document.getElementById('arrFilter_pf[CITY][RIGHT]').value=this.options[this.selectedIndex].value;" name="arrFilter_pf[CITY][LEFT]">
...
</select>[/CODE]
[CODE]<input id="arrFilter_pf[CITY][RIGHT]" type="hidden" value="" name="arrFilter_pf[CITY][RIGHT]"/>
<select onchange="document.getElementById('arrFilter_pf[CITY][RIGHT]').value=this.options[this.selectedIndex].value;" name="arrFilter_pf[CITY][LEFT]">
...
</select>[/CODE]