Single post pagination

It's possible to modify the content pagination with the content_pagination filter.

Here's a way to always display the content of the first page:

/**
 * Content Pagination: Always display the content of the first page
 */
add_filter( 'content_pagination', function( $pages )
{
    // Nothing to do if there's no pagination
    if( count( $pages ) <= 1 )
        return $pages;

    // Get the first page and remove it from the pages array
    $firstpage = array_shift( $pages );

    // Prepend the first page to all other pages
    foreach( $pages as &$page )
        $page = $firstpage . $page;

    // Add the first page again
    array_unshift( $pages, $firstpage );

    return $pages;
} );

Example:

Let's assume the content pagination is defined as:

aaa
<!--nextpage-->
bbb
<!--nextpage-->
ccc

Then the first page is:

aaa

The second one becomes:

aaa
bbb

and the third one displays as:

aaa
ccc

Update:

To have the first page as:

aaa
bbb

the second one as:

aaa
ccc

etc, then we only need to comment out this line:

// Add the first page again
array_unshift( $pages, $firstpage );