How to split the Message window in Outlook?

To split the reading/composing message window into two, you need to show the Split feature first, and then apply this feature. Please do as follows:

1. In the "Mail" view, double-click to open the email that you want to split its Message window.

Note: To split the composing message window for new, replying, or forwarding emails, please click "Home" > "New Email" to create a new email.

2. In the opening Message window, please click the "Customize Quick Access Toolbar" button > "More Commands". See screenshot:

3. Now the Outlook Options dialog box comes out with the "Quick Access Toolbar" tab selected. Please do as follows:
(1) Select "Commands Not in the Ribbon" from the "Choose commands from" drop-down list;
(2) Click to select the "Split" command in the command list box;
(3) Click the "Add" button;
(4) Click the "OK" button.

So far the Split button has been added to the "Quick Access Toolbar" at the top of the message window.

4. Put the cursor at the place where you want to split the message window, and click the Split button. See screenshot:

Notes:
(1) To disable the split windows, please click the Split button again.
(2) If you add the "Split" button in the reading message window, the button won’t appear in the composing message window (when creating a new email, replying, or forwarding emails), and vice versa.
(3) Both Outlook 2013 and 2016 support the Split feature.