BLP,
<html>
<head>
<script>
function func() {
if (document.getElementById("cbx").checked) {
if (document.getElementById("txt1").value == "" || document.getElementById("txt2").value == "") {
return alert("незаполнено");
}
document.getElementById("form").submit();
}
else {
document.getElementById("form").submit();
}
}
</script>
</head>
<body>
<form action="http://javascript.ru" id="form">
<input type="checkbox" id="cbx" onClick="this.checked ? document.getElementById('div').style.display='block' : document.getElementById('div').style.display='none'">
<div style="display:none;" id="div">
<textarea id="txt1"></textarea><textarea id="txt2"></textarea>
</div>
<input type="button" value="Отправить" onClick="func()">
</form>
</body>
</html>