Show Only Posts Owned By Logged In User + my post

First, I second Q Studio comment: check Tom McFarlin’s post with a warning about using pre_get_posts. More specifically, pre_get_posts works everywhere, including RSS and dashboard, and you might want to exclude your logic from such places.

That said, if I understood correctly what you are trying to do, you can achieve it by using author__in parameter of WP_Query in pre_get_posts.

In fact, the developer documentation have an example:

$query = new WP_Query( array( 'author__in' => array( 2, 6 ) ) );

Your code could be adapted like this:

add_action('pre_get_posts', 'wpse_382339_query_set_only_author' );

function wpse_382339_query_set_only_author( $wp_query ) {
    global $current_user;
    if( is_admin() && !current_user_can('edit_others_posts') ) {
    // 2 is just a placeholder for your user.
        $wp_query->set( 'author__in' , array( 2, $current_user->ID ) );
        add_filter('views_edit-post', 'fix_post_counts');
        add_filter('views_upload', 'fix_media_counts');
    }
}

Bear in mind that this allow users to SEE both their posts and yours. But not edit.

Let me know if it helps or you need more clarification