PST Merge to quickly join, add or merge multiple PST file into single PST file including calendars, contacts and address book and other email items. PST Merge allows to combine multiple PST files such as email folders, address book, contacts, journals, notes, calendars, tasks to one single PST file.

https://gallery.technet.microsoft.com/office/Merge-PST-Files-into-One-420741c3

Download PST Merge software which is smart solution that without any data loss merge multiple PST files in single PST file. This PST Merge tool takes less time in merging process and is one of the safe methods that give the advance option to combine PST files i.e. - Merge PST, Join PST and Merge Contacts as well as you can also Remove Duplicate Items and Exclude Deleted Items from PST file.

Read more at:  PST Merge software
 

If you're using Outlook Email client application where all your email saved in PST or Personal Storage Table file format then these two thechnique Split and Merge will definitely help you to come back from two major issues:

1. When have large size Outlook PST file.
2. When have large number of Outlook PST file in Outlook account to access.  

Split Technique: This technique will help you to break large size size Outlook PST file in some small PST files so that user can use data of large size file easily. 
Merge Technique: It will allow user combine and merge large number of Outlook email files in one single file.

You can take more information about both two technique from the link: Split and Merge Outlook PST file