How can I add a left Webpart when only a "Main" webpart is displayed?

Basically I created a Document Library. I minimized this library so users click into the section and get redirected to the files in another link.

Once in the files, I can add webparts only in the "Main" section since it's the only possible place. I want to add a "Right" webpart without losing the "Main" one. How can I add a right webpart so this works?

Thanks in advance.