Need Top-bottom split screen view on ipad.

As you have discovered, while slide-over views are supported, iPadOS does not support over-under split-screen multi-tasking in Portrait orientation. You are not the first to make this observation - and are unlikely to be the last.

Apple do invite submission of constructive comments and feature requests via its Product Feedback portal - this being the most appropriate channel for requests such as yours. For iPad and iPadOS:

Feedback - iPad - Apple

You are unlikely to receive a personal reply - but we understand that all feedback is logged and forwarded to appropriate teams with Apple.

If you'd like, please submit your request to Apple directly, using this link:--> Apple – Product Feedback

Regards

Giulio