how do i link the same gear into 2 slots and a default glamour?

[Iscah]

I hadn't thought of that. Yeah that'd probably work. But it'd be a very niche use case in which the user wants to use job-specific gear as the glamour and not just like, a company tabard on the SMN and a raincoat on the RDM or something.

That's still doable - you put the SMN/RDM gear on the BLM plate, not the one it belongs to. Then when you apply the plate to the BLM, they are now wearing [Generic Coat of Casting glamoured as Summoner Coat] and the system looks at that and says "sorry, you can't equip that Summoner Coat, so it's going to display as the Generic Coat" - which in this case is exactly what you want to happen.