<?php
add_action( 'init', function() {
    remove_post_type_support( 'code-snippet', 'excerpt' );
    remove_post_type_support( 'code-snippet', 'editor' );
} );

// Now create 2 fields for excerpt and content.
// I'll show a way to use code and a way to use Builder.

add_filter( 'rwmb_meta_boxes', function ( $meta_boxes ) {
    $meta_boxes[] = [
        'title'      => 'Code Snippet Before',
        'post_types' => 'code-snippet',
        'fields'     => [
            [
                'type' => 'wysiwyg',
                'id'   => 'content', // This is the must!
                'name' => 'Reason for snippet',
            ],
            [
                'type' => 'wysiwyg',
                'id'   => 'excerpt', // This is the must!
                'name' => 'Snippet Excerpt',
            ],
            // Custom style to overwrite the editor style set by WordPress.
            [
                'type' => 'custom_html',
                'std'  => '<style>#wp-content-editor-tools{background:none;padding-top:0;}</style>',
            ],
        ],
    ];

    return $meta_boxes;
} );

// This is for content
add_filter( 'rwmb_content_field_meta', function() {
    $post_id = filter_input( INPUT_GET, 'post', FILTER_SANITIZE_NUMBER_INT );
    return get_post_field( 'post_content', $post_id );
} );
add_filter( 'rwmb_content_value', '__return_empty_string' );

// This is for excerpt only.
add_filter( 'rwmb_excerpt_field_meta', function() {
    $post_id = filter_input( INPUT_GET, 'post', FILTER_SANITIZE_NUMBER_INT );
    return get_post_field( 'post_excerpt', $post_id );
} );
add_filter( 'rwmb_excerpt_value', '__return_empty_string' );