Assuming you are talking about archive file, import both of them into outlook. Manually move all messages into the one you want to keep, then delete the one that you moved everything out of
Open one of the files. Then, from the file menu, select import. select "from another program or file"; then select ".pst" file. Browse to your second outlook file and select it to complete the import.
You'll need to open both of the data files in Outlook, then move the emails, calendar, etc. from one to the other. Don't forget to compress after you do this.
For merging the two files use import and export from file menu. By this option it helps to import files from old document to new document using export option