I have tried to search in the View menu but cannot find anything that will allow this; I have also searched as best as I can in the Help file, but have not found any instructions there either;
Short answer, you can’t:
Bug 31481 - [RFE] Split pane in same window for side-by-side proof reading/ translating of 2 different files
What you can do is to go to Window → New Window to get a second instance of your document on a new window, and then arrange those two windows using your window manager.
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