As per example code of the tinyMCE, its convert all Textareas on the HTML pages into WYSIWUG editor, which is really stupid stuff by them though.
I have searched a lot about allowing tinyMCE for particular Textareas on the page and its really a common discussion topic on the forum but I haven’t got any satisfactory answer. I dont know why all answers are only for Drupal implementation.
Then I looked into tiny_mce.js and I found something like “spefic_textarea” and I searched into tinyMCE manual for this keyword (http://wiki.moxiecode.com/index.php/Special:Search?ns100=1&ns106=1&ns108=1&search=specific_textareas&searchx=Search) and found some help.
Heres is code which will solve your problem.
// General options
mode : "exact",
elements : "textarea_id1,textarea_id2", // just pass the ID's of your textarea for which you want to add the editor
// Your code related other settings, keep it same
Post originally taken from http://vikrant_labde.blogspot.com