"The easiest way is to open one in Outlook. Then select File > Import, and import the second pst into the first, checking the ""include subfolders"" button.
If these are not the 2003/2007 version of PST, but are from Outlook 2000 or earlier, they have a 2 GB limit and other issues, so I would recommend creating a new PST, and importing both archives into the new one."
