As long as I remember this was never possible, first time I tried it said me that I had to remove the glamour in order to use the item for a glamour. Or maybe it was an overlooking at the very beginning of glamour that was fixed.
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I manged to pull it off very early when glamour system was in placed since I wanted to keep the Gae Bolg Atma around without the glow when I hit novus. That was when I learn that glamour over-stacking doesn't works, and in between here and there I have glamoured gears with something else that's already glamoured too. It was only until recently that I found out it no longer work.
the way the game uses the vanity system in this game goes as followed (to my understanding)
Gear A dye colour= none.
Gear a glamored into gear B.
Gear a dye colour = Gear B.
Gear c dye colour = none.
Gear C glamored into Gear a's Gear b colour.
Gear c dye colour = gear B.
at least that how it assumes, however due to the fact the game would try its best to overwrite changes to gear a this causes some issues
Gear a= missing Gear C has no model, But stays into its Gear B but must stay what Gear A was, But theres no Gear a (essenchaly a looping paradox in the code which causes a lot of errors.)
Gear b= missing = the glamored item no longer excists, so the game says the piece must be glamored as Gear A, but since Gear a has only one step in this it retains Gear b's information, Gear C however is trying to become Gear a and the same paradox appieres,
when it comes to programming trying to tell something to become something that no longer excists causes small issues, when its a very short line of code gear a into gear b, the game can very quickly do this by using Gear b= item + colour, but with a 3 step process this confuses the game Gear c = gear a + colour + gear b + colour, since the two colours will contradict themselves, (even if they are the same,) that will cause an error, and the fact the game is trying to look up 2 items instead of one confuses it more on what takes priority.
hope this information assists onto why they may not be stacked.