You want a field in Infopath where seperate Rich Text fields can be displayed appended to each other?
I cobbled an example method together for you and attached - hope it gives you some ideas.
The method uses the fact that rich text fields/nodes contain XML (in XHTML format) themselves.
Using standard XML methods I select these XHTML nodes from two seperate rich text fields and then clone and append them to a third rich text field.