How to display a metabox and how to save/put values in in for text areas is beyond the scope of this question, but you asked how to have a rich text editable area, like the main content.
To do this, use the wp_editor function. WordPress will take your identifier, content and some options, and create a new TinyMCE instance with buttons etc
$settings = array( 'media_buttons' => false );
$content = get_post_meta( $post_id, 'kents-secondary-content-thing', true );
wp_editor( $content, 'kents-second-editor', $settings );
wp_editor will spit out the necessary html and css/js to display what you want.
Note that this will only work reliably in the admin area, frontend use of wp_editor will require some extra steps ( a good idea for a new question! ).
Also note that wp_editor will display a content editor area, but it wont save/validate that content, you still have to do that.
A sidenote:
Your code uses the_editor, a function that wp_editor replaced. When you find code from some website, always read it, and look up the documentation. How else will you catch the call to sendAllMyMoneyTo("0723-323...etc");
If you had done this, you would have seen this and saved yourself a lot of time:
Enabling WP_DEBUG and looking at your php log would also have told you the_editor was a deprecated function. Look these up as they will save you many headaches going forward