if I understand the question correctly, you change the url of form via javascript/jquery like so..
<select onchange="submitform('url1')"></select>
<select onchange="submitform('url2')"></select>
<script type="text/javascript">
function submitForm(url)
{
$("#formid").attr("action",url);
$("#formid").submit();
}
</script>
and if you do this on submit button, you may try something like so:
<input type="submit" onclick="return submitformviabtn()"></select>
<select onchange="submitform('url2')"></select>
<script type="text/javascript">
function submitformviabtn(e)
{
e.preventDefault();
$("#formid").attr("action","page.php");
$("#formid").submit();
}
</script>