Here's a situation in which you might want to have two separate sets of personal folders: You might create one top-level folder for messages about a specific project, while another top-level folder stores non-business mail.
By default, whenever you add a new set of personal folders, the default top-level folder name is Personal Folders. How original. It's like naming your cat, "Cat." Worse, it's confusing. The solution? Your common sense and creativity.
Change each new folder name from Personal Folders to something meaningful. Give your personal folders personal names.