Is it possible to make the RTF editor in control also an html editor? So if I allow user to create a document inside RTF editor, they can have choice to choose "edit source html" instead?
Such functionality should be possible to achieve with RadRichTextBox and HtmlFormatProvider.
However, please note that the RadRichTextBox control is closer to a word processor than an HTML editor, so the HTML support has its limitations. I suggest you refer to these articles for more detailed information: HTML Support, Import/Export settings.
When it comes to what approach to adopt, you can try using HtmlDataProvider in order to create a two-way binding between a RadRichTextBox control and a TextBox (for example) similar to this online example.