I suggest using the Merge PST Tool if you wish to easily combine Outlook PST files. Without any data loss, it merges Outlook PST files with ease. Both Microsoft Outlook and all Windows versions are supported. The PST file attributes that this application supports are notes, messages, emails, attachments, contests, tasks, and calendars. By merging ANSI and UNICODE PST files, Unicode PST files are created. It is possible to combine Outlook PST files without installing Microsoft Outlook. This tool is compatible with all Windows versions including Microsoft Outlook.
More information:- https://www.betavare.com/merge-pst/