<form name="myform">
<table border="0" cellspacing="0" cellpadding="5"><tr>
<td><textarea name="inputtext"></textarea></td>
<input type="radio" name="placement" value="append" checked> Add to Existing Text<br>
<td><p><input type="radio" name="placement" value="replace"> Replace Existing Text<br>
<input type="button" value="Add New Text" onClick="addtext();"></p>
</td>
<td><textarea name="outputtext"></textarea></td>
</tr></table>
</form>

<script language="javascript" type="text/javascript">
function addtext() {
	var newtext = document.myform.inputtext.value;
	if (document.myform.placement[1].checked) {
		document.myform.outputtext.value = "";
		}
	document.myform.outputtext.value += newtext;
}
</script>