U.Helper

<script type="text/javascript">
function open(){parent = window.open("http://..../tmpls/?a=tmpl;m=2;t=4","rrp","toolbar=0,width=400,height=400,left=400,top=400");}
</script>
<a href="javascript://" onclick="open();return false;">сюда кликай, а потом вводи текст</a>
<textarea onkeyup='window.parent.document.getElementById("tmpl").value = this.value;'></textarea>

function eText (e, ta) {
var d = e.value.length > 0 ? 0 : 1;
return document.getElementById(ta).disabled = d;
};

eText(this, 'id'); // id - id второго поля

<script type="text/javascript">
function eText (e, ta) {return document.getElementById(ta).disabled = e.value.length > 0 ? 0 : 1;};
</script>

<textarea id="area1" onchange="eText(this, 'area2')" onkeydown="eText(this, 'area2')" onkeyup="eText(this, 'area2')"></textarea><br>
<textarea id="area2" disabled="1"></textarea>