If you use the code then see result add new featured images like main featured image

WP ADD CUSTOM FEATURED IMAGES META BOX

<?php
    // ADD NEW FEATURED IMAGES META BOXES

    function listing_images_add_metabox () {
        add_meta_box( 'listingimagediv', __( 'Home Slide Image', 'text-domain' ), 'listing_images_metabox', 'portfolio', 'side', 'low');
    }
    add_action( 'add_meta_boxes', 'listing_images_add_metabox' );


    function listing_images_metabox ( $post ) {
        global $content_width, $_wp_additional_image_sizes;
        $image_id = get_post_meta( $post->ID, '_listing_images_id', true );
        $old_content_width = $content_width;
        $content_width = 254;
        if ( $image_id && get_post( $image_id ) ) {
            if ( ! isset( $_wp_additional_image_sizes['post-thumbnail'] ) ) {
                $thumbnail_html = wp_get_attachment_image( $image_id, array( $content_width, $content_width ) );
            } else {
                $thumbnail_html = wp_get_attachment_image( $image_id, 'post-thumbnail' );
            }
            if ( ! empty( $thumbnail_html ) ) {
                $content = $thumbnail_html;
                $content .= '<p class="hide-if-no-js"><a href="javascript:;" id="remove_listing_image_button" >' . esc_html__( 'Remove listing image', 'text-domain' ) . '</a></p>';
                $content .= '<input type="hidden" id="upload_listing_image" name="_listing_cover_images" value="' . esc_attr( $image_id ) . '" />';
            }
            $content_width = $old_content_width;
        } else {
            $content = '<img src="" style="width:' . esc_attr( $content_width ) . 'px;height:auto;border:0;display:none;" />';
            $content .= '<p class="hide-if-no-js"><a title="' . esc_attr__( 'Set listing image', 'text-domain' ) . '" href="javascript:;" id="upload_listing_image_button" id="set-listing-image" data-uploader_title="' . esc_attr__( 'Choose an image', 'text-domain' ) . '" data-uploader_button_text="' . esc_attr__( 'Set listing image', 'text-domain' ) . '">' . esc_html__( 'Set listing image', 'text-domain' ) . '</a></p>';
            $content .= '<input type="hidden" id="upload_listing_image" name="_listing_cover_images" value="" />';
        }
        echo $content;
    }
    add_action( 'save_post', 'listing_images_save', 10, 1 );

    function listing_images_save ( $post_id ) {
        if( isset( $_POST['_listing_cover_images'] ) ) {
            $image_id = (int) $_POST['_listing_cover_images'];
            update_post_meta( $post_id, '_listing_images_id', $image_id );
        }
    }

============================================================================

<?php 

    //SHOW CUSTOM POST FEATURED IMAGES

    while( have_posts() ): the_post();
    $image_id = get_post_meta( get_the_ID(), '_listing_images_id', true );
    $thumbnail_html = wp_get_attachment_image_src( $image_id, 'full' );
?>

    <img src="<?php echo $thumbnail_html[0]; ?>" alt="" />

<?php endwhile; ?>