You could probably do it by having a second, read only text box on the right (with its border set to zero width if you don't want it to be obvious that it's a text box), and the javascript necessary to copy the data would look something like this:
<script language="javascript">
function updateData() // copies the value of the left hand text box into the right hand one
{
document.form2.textbox.value = document.form1.textbox.value;
}
</script>
Then you could just add an onChange attribute to the text box, like this:
<form name="form1">
<input type="text" name="textbox" value="" onChange="updateData();">
</form>
Alternatively, you could get rid of the function and just have the code to copy the data as the onChange attribute, but this way makes it easier to modify the code to allow for multiple text boxes doing the same thing, and just looks neater in any case.