Espero que te sirva.
<script language="JavaScript">
function fieldPreview( evt ) {
formName01 = document.form01.inpName;
formName02 = document.form02.inpName;
formName01.value = formName02.value;
with ( document.all ) {
spanName.innerText = formName02.value;
}
}
</script>
<form name=form01 method=GET>
<input type=hidden name=inpName value="">
</form>
<form name=form02>
<input type=text id=inpName size=20 value=""
name=inpName onKeyUp="fieldPreview( event )">
</form>
<span id=spanName></span>
(lo acabo de encontrar por ahí)
