it just names it like archive or backup and defaults to where your user profile is, like for me:
C:\Users\<username>\AppData\Local\Microsoft\Outlook\backup.pst
you can change the location of where to create it, and name it. if you blow up and need to restore, use import/export feature in outlook. or just open the pst file also. it'll mount itself as a separate tree.
btw, i think you see two sets of personal folder since the default always has one there, and the default name of the pst you create is named that as well. you may have just left the default name when you created it, thus seeing two now.