Adding image URL to post meta

Following is the code I’m working with. It generates @2x copies of thumbnails and deletes them upon deletion of original images. I’m looking for a way to add the @2x image path to post meta. Which should be deleted upon deletion of original image.

function ci_retina_image_make_intermediate_size($file, $width, $height, $crop=false) {
    if ( $width || $height ) {
        $resized_file = image_resize($file, $width*2, $height*2, $crop, $width . 'x' . $height . '@2x');
        if ( !is_wp_error($resized_file) && $resized_file && $info = getimagesize($resized_file) ) {
            $resized_file = apply_filters('ci_retina_image_make_intermediate_size', $resized_file);
            return array(
                'file' => wp_basename( $resized_file ),
                'width' => $info[0],
                'height' => $info[1],
            );
        }
    }
    return false;
}

function ci_generate_retina_attachment_metadata( $metadata, $attachment_id ) {
    $attachment = get_post( $attachment_id );
    $file = get_attached_file($attachment_id);
    $old_metadata = $metadata;
    foreach ($metadata as $k => $v) {
        if (is_array($v)) {
            foreach ($v as $key => $val) {
                if (is_array($val)) {
                    ci_retina_image_make_intermediate_size($file, $val['width'], $val['height'], true);
                }
            }
        }
    }
    return $old_metadata;
}

add_filter('wp_generate_attachment_metadata', 'ci_generate_retina_attachment_metadata', 10, 2);

function ci_delete_retina_images( $attachment_id ) {
    $ci_metas = wp_get_attachment_metadata( $attachment_id );
    $ci_updir = wp_upload_dir();
    $ci_path = pathinfo($ci_metas['file']);
    $ci_path_name = $ci_path['dirname'];
    $ci_updir = wp_upload_dir();
    foreach ($ci_metas as $ci_meta => $ci_meta_val) {
        if ($ci_meta === "sizes") {
            foreach ($ci_meta_val as $ci_sizes => $ci_size) {
                $ci_original_filename = $ci_updir['basedir'] . "/" . $ci_path_name . "/" . $ci_size['file'];
                $ci_x2_filename = substr_replace($ci_original_filename, "@2x.", strrpos($ci_original_filename, "."), strlen("."));
                if (file_exists($ci_x2_filename)) {
                    unlink($ci_x2_filename);
                }
            }
        }
    }
}