The Thieves Guild hood can be worn with additional headgear, and I've tested wearing it with the Archmage's Robes, Vokun/Morokei and an Ebony Helmet.
I couldn't understand why this only worked with the Thieves Guild Hood until I looked at my character: the Thieves Guild Hood resembles more a skullcap/coif and can be worn underneath any other headgear.
I'm looking for similar skullcap/coif headgear (un-enchanted) so I can place my own enchantments on it. Is there anything else like this in all of Skyrim?
My end-goal is to wear simultaneously two headgears with custom enchantments.