igorfelix89,
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
</style>
<script>
document.addEventListener("DOMContentLoaded", function() {
var a = document.querySelectorAll('[name*="product"]'),
d = document.querySelector('[name="model"]'),
b = [];
[].forEach.call(a, function(c, a) {
b.push(c.value);
c.addEventListener("input", function() {
b[a] = c.value;
d.value = b.join("\n")
})
})
});
</script>
</head>
<body>
<textarea name="product_attribute[0][product_attribute_description][1][text]" cols="40" rows="5"></textarea><br>
<textarea name="product_attribute[1][product_attribute_description][1][text]" cols="40" rows="5"></textarea><br>
<textarea name="product_attribute[2][product_attribute_description][1][text]" cols="40" rows="5"></textarea><br>
<textarea name="model" id="model" cols="50" rows="5"></textarea>
</body>
</html>