First, on a separate desktop, open the window or app you'd like to use in Split View. This window or app will replace one of the windows or apps you are already using in Split View.
Now, go back to Split View, and put your mouse over the full-screen button on the app you'd like to replace. The drop-down will appear in two to three seconds. If it doesn't, move your mouse and try again.
Select "Replace Tiled Window."
Now, select the window or app that you would like to use in Split View.