Ciao ianstilo,
intendi una cosa tipo questa?
codice:
<script language="javascript">
function Controlla(ff){
var valCombo1 = ff.combo1.options[ff.combo1.selectedIndex].value
var valCombo2 = ff.combo2.options[ff.combo2.selectedIndex].value
if(valCombo1==valCombo2){
alert('VALORI UGUALI!!!');
return false
}else return true
}
</script>
<form onsubmit="return Controlla(this)">
<select name="combo1">
<option value="0">zero</option>
<option value="1">uno</option>
<option value="2">due</option>
<option value="3">tre</option>
</select>
<select name="combo2">
<option value="0">zero</option>
<option value="1">uno</option>
<option value="2">due</option>
<option value="3">tre</option>
</select>
<input type="submit" name="bottone" value="invia">
</form>