You can also try the KDETools PST Merge Software to join PST files into one PST files in MS-Outlook. Merge various PST files such as Emails, Calendars, Tasks, Notes and all. Capable in ANSI and also UNICODE formats. Join PST files in all Version of Windows and Outlook. Demo Version also used free to download. Preview option also an option to check before the joining process.
More Info: https://www.kdetools.com/pst-merge.html