Para ocultar/mostrar 2 textarea (o más) usa esto:
Código PHP:
<html>
<head>
<script type="text/javascript">
function mostrar(chk) {
ver = (chk.checked) ? 'visible' : 'hidden';
chk.form.txt1.style.visibility = ver;
chk.form.txt2.style.visibility = ver;
}
</script>
</head>
<body>
<form>
<input type="checkbox" onclick = "mostrar(this)" />
<textarea name="txt1" style="visibility:hidden"></textarea>
<textarea name="txt2" style="visibility:hidden"></textarea>
</form>
</body>
</html>
Saludos,
