U.Helper

<textarea name="message1" id="textBoard" onkeyup="document.getElementById('yadiv').value=this.value;"></textarea>
<textarea id="yadiv"></textarea>

<textarea id="message"></textarea>

onkeyup="document.getElementById('sex').value=this.value;

<textarea id="message"></textarea>

document.getElementById('message').onkeyup="document.getElementById('sex').value=this.value";

<textarea id="message"></textarea>

<script>
document.getElementById('message').onkeyup="document.getElementById('sex').value=this.value";
</script>

<textarea id="message2"></textarea>
<textarea id="sex2"></textarea>
<script>
document.getElementById('message2').onkeyup="document.getElementById('sex2').value=this.value";
</script>

var text = document.getElementById("message");  
  var cloneText = document.getElementById("sex");  
  text.onkeyup = function () {  
  cloneText.value = this.value;  
  }