Salve dopo essermi imbattuto su questo dilemma, vi riporto le soluzioni che mi hanno aiutato a risolvere il problema, spero siano utili anche a voi:

<script type="text/javascript">
tinyMCE.init({
    mode : "textareas"
});
/* This selects all textareas. */
</script>

<script type="text/javascript">
tinyMCE.init({
    mode : "exact",
    elements: "ta1,ta3"
});
/* This selects the textareas with the specified IDs.
   EG: <textarea id="ta1" ....
   NOTE: The comma after "exact" is required.
*/
</script>

<script type="text/javascript">
tinyMCE.init({
    mode : "specific_textareas",
    textarea_trigger : "mce_editable"
});
/* This selects all textareas for which you have added
   a "mce_editable" attribute to "true".
   EG: <textarea mce_editable="true" ....
   NOTE: This option is deprecated in favor of either of the next 2 options.
*/
</script>

<script type="text/javascript">
tinyMCE.init({
    mode : "textareas",
    editor_selector : "mceEditor"
});
/* This only selects textares for which you have added
   a class attribute with a value of "mceEditor".
   EG: <textarea ....
   NOTE: The documentation does not say you need the "mode"
   configured but you do.
*/
</script>

<script type="text/javascript">
tinyMCE.init({
    mode : "textareas",
    editor_deselector : "mceNoEditor"
});
/* This selects all textareas except for those which you have added
   a class attribute with a value of "mceNoEditor".
   EG: <textarea ....
*/
</script>