Top Sidebar, above right & right inner sidebars

Hello!
I'm using the latest atahualpa-build. I use the 2 Sidebars (Right sidebar + Right Inner sidebar) and the central column of the topic.

Now I want a third one above those two, one that spans both of them. I don't want the third one to be above the header or something like that, it should still be a sidebar.

Example picture

How do I do this?

Sorry if this has been brought up earlier, I couldn't find and/or understand any other threads about it.