I have two textareas that I load when the page loads, these two textareas are in the same table row. My tinyMCE looks like this
tinyMCE.init({
Give them different ids. Tinymce editor id is equal to the textareas id; if no textarea id is present "content" is used as default. In your case for both textareas. Because there is already a tinymce editor with id "content" when the second textarea is being converted into a tinymce editor instance - it fails.
This has now been fixed by upgrading to the current version of tinyMCE (3.5b2) - as per:
http://www.concrete5.org/community/forums/customizing_c5/firefox-11-tinymce-issue-workaround-and-fix/
In my code, it was not fixed by giving different ids to the multiple text areas (Thariama's suggestion above).
If you don't know how to figure out your version of tinyMCE, see How do I find out what version of TinyMCE I am running?