Quote:If you have received a long email—or you’re writing one—Outlook lets you split the mail vertically into two panes, so you don’t have to scroll quite so much to reference different parts of the mail.

If you’ve ever had to deal with a long email full of replies from different people, you’ll realize how handy this little feature is right away. You could use the top pane to keep an older part of the conversation in view so that you can refer to it while using the bottom pane to type your reply. And of course, that’s only one use. You’ll surely find many others.

To split the mail, click in the Reading Pane, or double-click the mail to open it, and then click Ctrl+Alt+S. This will put a horizontal splitter bar in the middle of the mail, with a separate scroll bar in each pane.

Full reading: https://www.howtogeek.com/398888/quick-tip-how-to-split-an-outlook-mail-into-2-separate-panes/