I agree with the answer above, but suggest adding the point to check 'do not import duplicates' underneath where you supply the path to the file. (can't edit questions or comments yet, thus the format of a new answer.)But in Outlook 2000 and below version, if you'll try to import two archives data to a newly created PST file, it may take unspecified time. So to save your time you can go through alternative solution.
Their migration can be fraught with issues and exceptions thrown from password-protected, corrupt or in-use files, as well as issues from files created by staff who have left the organization.
Let's say you want to have all information finally in file 'C'.
Start 'Synch Pst for Outlook' and choose on the left side file 'A' and on the right side file 'C'.
If PSTs are posing a problem in your organization, then you should consider eliminating PSTs entirely by migrating and consolidating PSTs to more cost-effective platforms such as Office 365, Exchange 2013 or 2016, or a cloud or on premise archiving solution like Netmail Archive.
We Can Help With such a large amount of duplicated, dispersed information, legal departments struggle to ensure they have the right data to defend cases and compliance.