<script language="Javascript">
function abgleich() {
document.forms[0].textfeld2.value=document.forms[0].textfeld1.value;
}
</script>
<form action="index.php" method="post" name="write">
<textarea name="textfeld1" style="width:100%;" rows="35">
<textarea name="textfeld2" style="width:100%;" rows="35">
<input onClick="abgleich();" name="abgleich" value="1" >
</form>