As you mentioned above to display the same files at same time, you can open PDF on two different Windows.
For that, you will need to uncheck the box for "Open documents as new tabs in the same window" under the preferences.
Launch the application and go to Edit menu > Preferences > General and uncheck the box.
Once done, click OK at the bottom to save the changes and then relaunch the application.
Now try to open the documents to view at the same time.
Let us know if you are referring to something else.
Or, more easily, drag the tab out of the window.
Thank you Jane-e.
At first, i did not understand, I don't know how to t(ranslate "tile".
But your screen-copies are perfect, so I am OK.
I'll remember the french word for tile : mosaïque.