Is it possible to split the screen horizontally rather than vertically?

This can be done with SplitView. You need to use the keyboard shortcuts to move windows up or down.

Press Windows key + Up arrow (or number 8 on the number pad) to move a window to the top half of the screen.
Press Windows key + Down arrow (or number 2 on the number pad) to move a window to the bottom half of the screen.