You can only operate one outlook profile at a time and so the only way to access the “old” .pst is by importing it using the inbuilt process.
If you want to remove your forgotten then Microsoft released a utility called pst19upg.exe to upgrade old versions of *.pst (Personal Folders) files so they would work with a later version. One side effect of this was that it removed the password from the pst file. So if you password your pst and forgot the password, you can use this utility to remove the password so you can access your email.
You need to use two utilities to remove the password from an ANSI pst (old format):
scanpst.exe – This is installed when you install Outlook. It should be in your Office folder, under Program files- usually C:\Program Files\Microsoft Office\OfficeXX (where XX is the version number) or use Windows Search to find it.
If this method doesn’t work or you have forgotten the password on an Unicode pst, you’ll need a commercial password cracker program.