In the editor, click on View on the top ribbon. There will be a Split Document section: click on Left/right. Click back on Editor on the top ribbon and open one of the files. Click on the other plane to activate it and then you can open a different file there.
If the purpose is comparison of files, you might want to consider a different approach. In the editor when the Editor ribbon (the default one) is active, just to the right of the Save icon there is "Compare". Click on "Save and Compare with" to get to a screen that allows you to select two files. When you activate, then a file "diff" operation will be done, showing you the differences between the files.
