mephistop,
<!DOCTYPE html>
<html>
<head lang="en">
<meta charset="UTF-8">
<script src="//code.jquery.com/jquery-1.11.3.min.js"></script>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.3/jquery.min.js"></script>
<title></title>
</head>
<body>
<textarea> </textarea>
<input class="myInput" id="duble" type="text" />
<script>
$(document).ready(function () {
$('textarea').keyup(function () {
var text = this.value;
var l = $('#duble').val().length;
if(l > 10){
$('#duble').val($('#duble').val());
return;
}
$('#duble').val(text);
})
})
</script>
</body>
</html>