гавнокод
<div>
<div id="block1"></div>
<div id="block2"><!-- content begins -->Some Content <!-- content ends --> content continue..</div>
</div>
<script>
var str = block2.innerHTML, pos = str.indexOf("ends -->")+8
block1.innerHTML = str.substr( pos );
block2.innerHTML = str.substring( 0, pos );
</script>