Since one div is initially hidden, you can simply call toggle for both divs:
<a href="javascript:void(0);" id="forgot-password">forgot password?</a>
<div id="login-form">login form</div>
<div id="recover-password" style="display:none;">recover password</div>
<script type="text/javascript">
$(function(){
$('#forgot-password').click(function(){
$('#login-form').toggle();
$('#recover-password').toggle();
});
});
</script>