Adverterio,
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Test Frontend</title>
<script src="//code.jquery.com/jquery-1.11.3.min.js"></script>
<style>
</style>
</head>
<body>
<select name="first" id="first">
<option value="1">Text 1</option>
<option value="2">Text 2</option>
<option value="3">Text 3</option>
<option value="4">Text 4</option>
</select>
<select name="second" id="second">
<option value="1">Text 1</option>
<option value="2">Text 2</option>
<option value="3">Text 3</option>
<option value="4">Text 4</option>
</select>
<span id="result"></span>
<script>
$('select').on('change', function () {
var res = ( +$('#first').val() ||0) + (+$('#second').val()||0);
$('#result').html( res)
})
</script>
</body>
</html>