[j0hnik]

<textarea name="product_attribute[0][product_attribute_description][1][text]" cols="40" rows="5">231213</textarea>
<textarea name="product_attribute[1][product_attribute_description][1][text]" cols="40" rows="5">123213213</textarea>
<textarea name="product_attribute[2][product_attribute_description][1][text]" cols="40" rows="5">213213123</textarea>

<textarea name="model" id="model" cols="50" rows="5"></textarea>

<button id="btn">скопировать</button>

	<script>
document.querySelector('#btn').onclick = e => {
document.querySelector('#model').innerHTML = [].map.call(document.querySelectorAll('[name^="product_attribute"]'), el=>el.innerHTML).join('\n');
};
	</script>