I’d like to post my solution since this thread is one of the first results on google and could be useful to many.
The point is that WP compiles a configuration object for each editor and it could be used only once in a webpage. The trick is to store the config object, manipulate it to manage your ajax editor and trigger tinymce.init()
Code for the webpage
<?php // create hidden fake editor echo ''; wp_editor('', 'editor_id'); echo ''; ?>
Code for ajax response
<?php $eid = 'NEW_editor_id'; wp_editor('', $eid); ?>
Simple, isn’t it? But has been a real nightmare to figure everything out without using absurd resources