"base item and projected item are equippable by the same class or job"

[Zourin]

If you glamour a metal chestplate (GLA PLD MAR WAR) on top of a peice of crafter gear ("All Classes"), then the platemail will only show when you are a PLD/MAR/GLA/WAR. Realistically, you'd be doing this the other-way-around.

In short, there has to be some point of class overlap for the glamour to work. The glamour will fail if one peice is LNC/DRG and the other is PUG/MNK, for example.. but it will work if one or the other peice is All Classes, Disciple of War, or otherwise includes the class you're trying to glamour with.