DrewSherwood,
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
</style>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.12.4/jquery.min.js"></script>
<script>
$(function() {
var end = $(".end");
$(".start").html(function(index, html) {
var el = end.eq(index);
$(this).html(el.html());
el.html(html)
});
});
</script>
</head>
<body>
<div class="start" id="firstID">
<p>One</p>
</div>
<div class="end" id="firstID">
<p>Two</p>
</div>
<div class="start" id="secondID">
<p>Three</p>
</div>
<div class="end" id="secondID">
<p>Four</p>
</div>
</body>
</html>