I want to build a Folder or Sidebar into a Folder. It should work like this:
1. Open Folder
2. Click on Sidebar/Folder
3. Folder/Sidebar ist getting opened into the Folder. The first Folder should not get closed. So both folders (or the folder and the Sidebar) should been opened at the same time. But I don't wan't that both are opened with one click, because I want the second folder only as an extension.
simply said: I want that the first Folder doesn't get closed if I open the second one. They should overlap.

How can I do this?