Display two previous posts on single.php

I hope there are multiple ways to accomplish this, but I suggest you use the following simpler function for this purpose. This is an attempt to grab a post for the current post which excludes it and the previous post:

// A custom function in your functions.php file
function get_related_previous_post($previous = 0) {

    global $post; // take current post object
    $category_ids = array();
    $categories = get_the_category($post->ID); // current post categories
    
    if ($categories) {
        foreach ($categories as $individual_category) {
            $category_ids[] = $individual_category->term_id;
        }
    }

    $args = array(
        'post__not_in' => array($post->ID, $previous), // exclude posts
        'posts_per_page' => '1',
        'orderby' => 'date',
        'order'   => 'DESC',
        'category__in' => $category_ids // apply same categories 
    );
    $prevprev_id = 0;
    $my_query = new WP_Query( $args );
    if ($my_query->have_posts()) {

        while ($my_query->have_posts()) {
            $my_query->the_post();
            // Do your stuff
            $prevprev_id = $post->ID;
        }
    }
    wp_reset_postdata();
    return get_post($prevprev_id);
}

Call the function in your single.php file like:

if (!$next) {
    // Make sure there's a previous post
    if ($prev) {
        $prevprev = get_related_previous_post($prev->ID);
    }
}