you’ve got a lot of options. some of which are:
to toot my own horn, i have used WP Alchemy to create repeating sortable tinyMCE editors with all the code available here
http://www.kathyisawesome.com/426/multiple-wordpress-wysiwyg-visual-editors/
there is also the http://www.advancedcustomfields.com/ plugin. you can definitely have multiple instances, but you can’t add dynamic blocks on the fly.