To split the reading/composing message window into two, you need to show the Split feature firstly, 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 in 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) For disabling the split windows, please click the Split button  in the Quick Access Toolbar again.
(2) If you add the Split button  in reading message window, the button won’t appear in composing message window (when creating new email, replying or forwarding emails), and vice versa.
(3) Both Outlook 2013 and 2016 support the Split feature.