I’m using a large iPad Pro in portrait orientation (ie, screen taller than it is wide). I’d like to split the screen so that Safari is on the top half and my note taking app is on the bottom half. It doesn’t seem to allow it. Is it not possible or am I missing a gesture?
This is currently not possible — apps can only be placed to the left or right, not at the top or bottom, regardless of device orientation.
Drag an app out of the Dock to make a Slide Over or drag it to the right or left edge of the screen to make a Split View
You do get to choose if you have the device in landscape mode or portrait mode so if you need to prioritize for extra height, you can. In your case, prioritizing for extra width isn't possible currently.
You can provide feedback to Apple about iPad here if you have a great use case for top/bottom split in the future: