Apple's Split View system on iPad allows you to open Ulysses and a second app, or even two Ulysses windows, side by side.
Of course, you can have multiple Ulysses windows open side by side on your Mac. However, there is also the option to split the editor directly in the app, which makes working with two texts even more convenient.
You can easily recognize the text you are currently editing by its highlight. However, you can also switch off this feature by unchecking “Highlight Active Editor” in the Second Editor menu, accessible via the rings icon. Here, Ulysses also allows you to swap both editors, and to revert to the one editor view by closing the second editor.
Would you rather have a horizontal split than a vertical one? No problem, select the option “Second Editor on Bottom” in this menu.
Do you want to compare two texts? Hold ⌥ (option) during scrolling to move both editors simultaneously.
You can switch between the two editors without lifting your fingers from the keyboard: Use the shortcuts ⌘⌥→ and ⌘⌥← (command-option-right/left arrow).
It's up to you how much space you give each of the editors on your screen: drag the divider to left or right as you see fit. To split the available space evenly between both editors again, double-click the divider.