PST Merger tool
You can easily combine multiple PST files, I would suggest you go for try Merge PST Tool. This tool is to merge multiple PST files by email, contacts, tasks, journals, notes, and calendars Option. It merges any PST files as there is no limitation and It allows to removes duplicates PST files or merges unlimited PST files. It supports all versions of Windows and Outlook like 2003, 2007, 2010, 2013, 2016, 2019, etc.
Visit here for more info: http://www.download.adwages.com/outlook-pst-merge/