Cannot equip a secondary staff as a monk

Apparently I discovered from my fellow monks that a monk can equip a secondary staff alongside a quarterstaff (3 different monks told me that).
They also sent me an image of how their gear look like, and it is clearly shown that they have this small box for a staff near the main weapon which I don't have at all.

Not sure what caused it, since I didn't have it from the day I opened my character. I thought it might be gear/passive related but honestly one of the monks is a lower level than me and the other one is way higher than me so don't think this is the case.

If needed, can supply the images by email or something since it seems like I can't upload it here.