This has been a long-standing request but not yet implemented. As of LO Version 5, Writer is not among the suite's apps that can split a window. From LO Help:
Documents of LibreOffice Calc, LibreOffice Draw, and LibreOffice Impress can be split horizontally and vertically into separate views. Each view can show other parts of the document. Using the mouse, you can drag a dividing line from the scrollbar into the document.
LO Version 6 is now out (I don't have it yet), but they don't list this as a new feature.
You can open multiple windows, including the same document, but those views are not synchronized (changes in one aren't reflected in the other). Each window reflects the version of the document as last saved. So if it is adequate for your purpose to refer to old, static contents in a second window, that can be a partial solution.
But remember, any changes you make in one window and save will supersede what you're looking at in the other window. If you work in two areas of the same document in two different windows, you risk losing half of your work when you save either one.