Hi, merging PST files into one is possible in 2 ways. One is manual way and another one is by using a 3rd party PST merge tool.
Here are the steps for you for the manual solutions.
1. Create a New PST File:
•Open Outlook and go to File > Account Settings > Account Settings.
•Click on the Data Files tab.
•Click Add to create a new PST file. Choose a name and location for this file.
•Click OK to add it to Outlook.
2. Import the First PST File:
•Go to File > Open & Export > Import/Export.
•Select Import from another program or file and click Next.
•Choose Outlook Data File (.pst) and click Next.
•Browse for the PST file you want to import, select it, and choose Replace duplicates with items imported.
•Choose the new PST file as the destination and click Finish.
•Wait for the import to finish.
3. Repeat for Other PST Files:
Repeat the import steps for each PST file you want to merge into the new PST file. Each time, select the same new PST file as the destination.
If you want a reliable and easy solution to avoid the complexity of technical aspects, you can try this PST Merge Tool which can merge your 2 PST files into one by maintaining the data integrity, since its an MVP recommended tool. Here is the details: https://www.stellarinfo.com/merge-outlook-pst-files.php