How do you open 2 (or more) different documents (written pages) simultaneously?
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I’m not sure what you mean, I regularly open multiple documents … they appear as separate windows.
A trick that I use is to make sure that none of the several windows is “full screen” and that each one is offset a little from the others. Then I don’t have to keep track of which one is which in the Task Bar at the bottom, but can see which one I want to work on next, just like slightly-scattered pieces of paper on my desk. Of course this is not possible on a small-screen device like a tablet.
Writer doesn’t have “View side by side”, if that’s what you ask.
But you can use operating system (which you didn’t specify).
Windows has “Aero Snap”. Windows logo key+ left or right arrow key. OR drag by title to left and right screen border.
When you open the file dialog, either hold down the ctrl key and then click on the desired files one by one, or, hold down the shift key click a file and then you can skip several files in the list and click a file. the files between these two selected will be selected along with the two you clicked on.