Watch this video. (To view it larger than it appears here, click the 'Play' button and then click the 'Full Screen' icon icon at the bottom right). There should be something similar on your own keyboard:
My Android tablet keyboard looks nothing like the one in Buenchico's link and there is no similar setting.
JJ, go in to Settings - Keyboard and tell us what keyboard your tablet is using. It might be GBoard, Swiftkey or something else. At least that would give us a starting point
I have just discovered that my Android tablet did not have Google keyboard installed (GBoard). I have now installed it and found how to enable/disable the split screen.
At the very top of the keyboard is a series of icons Smiley - T (text) and then a square representing a keyboard.
Tap that and the screen will revert to normal.