Considering the following HTML:
<form id="upvoteForm" method="post" action="/post/upvote">
    <input type="text" name="post_id" id="post_id"/>
</form>
<form id="downvoteForm" method="post" action="/post/downvote">
    <input type="text" name="post_id" id="post_id"/>
</form>
<input type="hidden" id="_postid" value="1"/>
I'm trying to set the two input fields with the name post_id to to value from _postid using this JavaScript and jQuery:
$(document).ready(function() {
    $('#post_id').val($('#_postid').val());
});
However, as you can see in this jsFiddle, it's only setting the value of the first one. How do I set the value of both of them? I thought the selector would end up grabbing both.
Now, I realize you might be wondering why I have two forms on this page. The basic reason is I have button inputs that I've styled the way I want but then I use the onclick to call the submit of the appropriate form here. I am ultimately going to be leveraging AJAX here, but that's coming later.

idattribute is always supposed to be unique.