Best practice for collapsible submenus?

I have a website navigational menu:

• Lorem
• Ipsum <
  • Situs
    • Pratius
    • Quisque
  • Dolor
• Duis
• Proin <
  • Folios
    • Artius
    • Zalto
  • Ramo
• Neque

What should be the best practice for users; if a user opens up the Ipsum submenu, then opens up the Proin submenu, should the Ipsum submenu close before the Folios submenu opens up, or should it leave the previous open one and simply open up the new one?

Also, are there any scenarios in which the user would want to have a list of collapsable submenus that would open up without closing the sibling submenus?