If your WYSIWYG-Editor isn´t able to do this, you´ll need to add event listeners to both sites. Key event listeners at each form element in the form site and one for tab in the frame site. You will also need an custom event in the main page, which you can trigger from inside frame. This event then should take focus from the frame to the next form element. Also your key listener should take manually the focus to the next field by preventing the default action.
As far as I know, TinyMCE uses per default an (i)frame for the editor content. Maybe you should also look at TinyMCE for a simplier solution.