when reading mail in iPad Pro the screen divide into 2 screens. How do I make it remain in one screen?

Have 3 year old iPad Pro. While in mail and hot keying a news site my screen divides in half and don't know how to make it go back to full mail screen. The half screen is to small to read articles without magnifying which mead 2-3 words a line. How can I stop it from doing that automatically.