As you say, the Text property gets just the text and not the markup. When you read the documentation for the RichTextBox class, you'd have noticed that the Rtf property is the one you use to get the markup and text.

I've never actually tried to combine the contents of multiple RTB controls. I'm not sure if it's as easy as simply concatenating the Rtf properties because each one probably contains its own root node. I've seen mention of classes designed to work with RTF data in code with no UI. I have no experience with any of them or even know where to find them, but if you search around you may be able to find one or more and they may be able to do what you want.