How to merge two or more WordPress posts?

The requirements you are describing require much more of an explanation than is really suitable here. A simple solution might be to mark the duplicate posts as duplicates via a meta box on the post screen. This meta box would have one input: A reference to another post.

Check out this tutorial on how to build a custom meta box.

Suppose you’ve set up a meta box that will allow the user to mark a post that is a duplicate of another post. Say the duplicate post is marked with a meta key _duplicate_of_post, whose meta value is the canonical post’s post ID.

You can then on filter the_posts filter to pull together any duplicates, folding the the canonical post together with any of its duplicates:

function my_the_posts_filter( $posts, $query ) {
    // Only operate on the main query.
    if( !$query->is_main_query() )
         return $posts;

    // Store canonical posts here. 
    $canonicals = array();

    // Store the duplicates here, keyed by canonical post ID
    $duplicates = array();

    foreach( $posts as $post ) {
        if( $dupe_id = get_post_meta( $post->ID, '_duplicate_of_post', true ) ) {
           $canonicals[] = get_post( $dupe_id ); 

           $duplicates[$dupe_id] = get_posts( array( 'meta_key' => '_duplicate_of_post', 'meta_value' => $dupe_id, 'posts_per_page' => -1, 'post__not_in' => $post->ID ) );
        }
        else {
           $canonicals[] = $post;

           // Get all posts who have this marked as duplicate
           $duplicates[$post->ID] = get_posts( array( 'meta_key' => '_duplicate_of_post', 'meta_value' => $post->ID, 'posts_per_page' => -1 ) );
        }
    }

    // Append all of the other post content to the canonicals separated
    // by two newlines
    foreach( $canonicals as $post )
       $post->post_content .= array_join( "\n\n", wp_list_pluck( $duplicates[$dupe_id], 'post_content' ) );

    return $canonicals;
}
add_filter( 'the_posts', 'my_the_posts_filter', 10, 2 );

One caveat in implementation of the post reference meta box is that you’d have to implement the logic to ensure that you can’t set a post as a duplicate post is if it’s already marked as a “master” post by another post. Without that restriction, the example above will break in unexpected ways.

Update

You can also hook on save_post, combine, then trash posts marked as duplicates:

function my_save_post( $post ) {
    // Don't do on autosave
    if( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) return;

    if( isset( $_POST['_duplicate_of_post'] ) && ( $dupe_of_id = (int) $_POST['_duplicate_of_post'] ) && ( $source = get_post( $dupe_of_id ) ) ) {  
       // Add this post's content to the canonical version               
       $update = array( 'ID' => $source->ID, 'post_content' => $source->post_content );
       $update['post_content'] .= "\n\n" . $post->post_content;

       // Update canonical
       wp_update_post( $update );

       // Trash this post
       wp_trash_post( $post->ID );
    }
}
add_action( 'save_post', 'my_save_post' );

This is the most basic of examples. It’s expecting a form field on the edit post screen named _duplicate_of_post which contains the canonical post’s ID. Note that I’m using ONLY the WordPress API, which is something you should always aspire to do while developing theme and plugin code.