Outlook doesn't limit you to having only one Personal Folders file, although—in most cases—you don't need more than one. If you do have more than one, you must designate which one you want Outlook to use as the default for items you create and receive.
Note
One possible purpose of having a second Personal Folders file is to have a place where you can experiment with Outlook and learn how to use it. Use the primary Personal Folders file for your normal work; have a second Personal Folders file in which you can work without the risk of damage to your work items.