my question is about
Can I combine two Jumbo XL sheds together instead of buying the extension kit?
Yes, you can. You can add the front part of the shed to the existing unit, as an Extension kit, and use the rest to create a XL Vertical Shed (57"Wx44"Dx80"H) - assembly instructions available from the site.
Does the XL shed come with shelves?
No, brackets are designed to be used for a 1" x 12" piece of wood and you can design your own length of shelf.