[ksa]

Например, при выборе test11 и test21 срабатывал alert.

Как вариант...

<select id="test1">
	<option></option>
	<option value="test11" id="test11">test11</option>
	<option value="test12" id="test12">test12</option>
	<option value="test13" id="test13">test13</option>
</select>
<select id="test2">
	<option></option>
	<option value="test21" id="test21">test21</option>
	<option value="test22" id="test22">test22</option>
	<option value="test23" id="test23">test23</option>
</select>
<script type='text/javascript'>
(function(){
	var o1=document.querySelector("#test1");
	o1.addEventListener("change", test);
	var o2=document.querySelector("#test2");
	o2.addEventListener("change", test);
	function test(){
		if (o1.value=='test11' && o2.value=='test21') {
			alert('Ok');
		}
	};
	test();
})();
</script>