Иллюстрация...
one.js
if (window.two) {
two();
} else {
alert('Not function two()...');
};
function one() {
alert(1);
};
two.js
if (window.one) {
one();
};
function two() {
alert(2);
};
three.js
if (window.one) {
one();
};
if (window.two) {
two();
};
Ну и ХТМЛ...
<!DOCTYPE html>
<html>
<head>
<script src="one.js"></script>
<script src="two.js"></script>
<script src="three.js"></script>
<!--
<script src="http://code.jquery.com/jquery-latest.js"></script>
<link rel="stylesheet" type="text/css" href="tmp.css" />
-->
<style type="text/css">
</style>
<script type="text/javascript">
</script>
</head>
<body>
</body>
</html>