Apple did not introduce a horizontal split view in portrait mode in iPadOS 15 and will probably not care to do so in the coming years.
Introduce a top-bottom horizontal split view in portrait mode, so that you can show a full-width part of a text book / lecture / exercise sheet at the top and your notes on the bottom on the 12.9 inch iPad.
Right now you need to switch between tabs. Every. Single. Time.
This is an essential feature for every student and a must-have for GoodNotes!