CKeditor returning 403 when submitting certain html tags
This would be the result of your mod_security rules. Depending on how strict they are they help better protect scripts from being hacked through vulnerabilities, generally those exploited via POST's.
As I understand you are trying to add something to your second textarea. And CKEditor removing some "unsafe" tags. I won't be very secure, but this can help you:
config.extraAllowedContent = '*{*}';You will add this to your config.js. This code provides you to add anything you want. And CKEditor won't delete "unsafe" tags.