Heve you tried an onkeypress functions?
Something like this:
function copy_data(val){
var messageText = document.getElementById(val.id).value
document.getElementById('message').innerHTML = messageText;
}
And then in your text area add:
<textarea name="original text" onkeypress="copydata(this)" >
This will copy the data from the current textarea to a text area with the ID message ...