Both of those items you listed are class-specific. Currently, they MUST be looted on their relevant class in order to be added to the appearance library.
So did a Rogue loot the Fanged Slayer’s Helm AND did a Warrior loot the Titanic Onslaught Pauldrons?
Any items that are not class-specific can be looted and learned by any class.
Blizzard has made mention of adding the ability for any class to loot and learn an appearance during a later patch in The War Within.
link: https://us.forums.blizzard.com/en/wow/t/collecting-appearances-in-the-war-within/1915253