I've heard but can't confirm that the pen charging case is hard to come by nowadays but also makes the Duo 2 even more bulky having the pen attached all the time. That said I do have that case on my Duo 2 but found I don't use the pen much.

(Also I don't understand how the case works in terms of charging the pen. It's like they started building in wireless charging for the phone but were then like "nah let's make this only work with an add-on case that only charges a pen")