You cannot open two full copies of the same object at once. Each object can only be open in one main or side panel tab at a time. Opening a page in the side panel and then choosing Open as page moves it to the main area and removes it from the side panel.

Embed views are different. An embedded block from a page can appear in the side panel while that same page stays open in the main view.

If you want an outline on one side and a draft on the other, a good workaround is:

• Create a separate page (for example, just for your outline or notes).
• Open your main page in the main area.
• Open the outline page in the side panel.
• When you are done, copy the content back into the main page and delete the temporary page if you like.

If you would like full duplicate tabs for the same object, you can upvote or comment on "Allow the same object to be opened in the side panel".