you would need to hand code javascript to accomplish this.
assuming the name of the form is "form" and the name of the select lists are "select1" and "select2", the javascript function would look like this:
<script type="text/javascript">
function checkform(form) {
retval = true;
if(document.form.select1.value == document.form.select2.value) {
alert("please make unique selections");
retval = false;
}
return retval;
}
</script>
then add an onsubmit call to the form tag:
<form id="form" name="form" method="post" action="" onsubmit="return checkform(this);">