iPadOS multitasking supports side-by-side split-screen in Landscape screen orientation only. You’ll perhaps note that the individual App windows, in side-by-side view, are actually the equivalent condensed “mobile” portrait view that you would see when using an iPhone.
iPadOS multitasking does not support an over-under multitasking view in Portrait screen orientation - perhaps as re-scaling an App’s native portrait view to a landscape aspect ratio is not supported by many Apps.
With introduction of iPadOS16, newer models of iPad Pro and iPad Air now support Stage Manager, offering a much more flexible multi-window environment - but this too is fundamentally limited to being used with the iPad in Landscape orientation.
Apple do invite submission of comments and feature requests via its Product Feedback portal:
Feedback - iPad - Apple