Multipaging posts from template

Hi, I’ve been trying to manually multipage posts with no result. I explain, instead of writting in the post_content the “< !–nextpage–>” tag, i’m trying to code it inside:

For example:
$content = $post->post_content."< !--nextpage--> Extra info < !--nextpage--> Gallery of the post";

So my intention is that the posts look like this:
Content domain.com/cat/post-name
Extra info domain.com/cat/post-name/2
Gallery domain.com/cat/post-name/3

I’ve found the function that divides the pages, after modifying it I found no results, the pagination is displayed but when you try to access domain.com/cat/post-name/2 it directly redirects you to domain.com/cat/post-name/ as if not second page was found.

function setup_postdata( $post ) {
	global $id, $authordata, $currentday, $currentmonth, $page, $pages, $multipage, $more, $numpages;

	$id = (int) $post->ID;

	$authordata = get_userdata($post->post_author);

	$currentday = mysql2date('d.m.y', $post->post_date, false);
	$currentmonth = mysql2date('m', $post->post_date, false);
	$numpages = 1;
	$multipage = 0;
	$page = get_query_var('page');
	if ( ! $page )
		$page = 1;
	if ( is_single() || is_page() || is_feed() )
		$more = 1;
	//MY MODIFICATION
	$content = $post->post_content."<!--nextpage--> Page 2";
	//END OF MYMOD
	if ( false !== strpos( $content, '<!--nextpage-->' ) ) {
		if ( $page > 1 )
			$more = 1;
		$content = str_replace( "\n<!--nextpage-->\n", '<!--nextpage-->', $content );
		$content = str_replace( "\n<!--nextpage-->", '<!--nextpage-->', $content );
		$content = str_replace( "<!--nextpage-->\n", '<!--nextpage-->', $content );
		// Ignore nextpage at the beginning of the content.
		if ( 0 === strpos( $content, '<!--nextpage-->' ) )
			$content = substr( $content, 15 );
		$pages = explode('<!--nextpage-->', $content);
		$numpages = count($pages);
		if ( $numpages > 1 )
			$multipage = 1;
	} else {
		$pages = array( $post->post_content );
	}

	do_action_ref_array('the_post', array(&$post));

	return true;
}

I need help!