What to do with multiple invulnerable saves?
(Note: I am assuming you are playing 9th edition here since it is the latest ruleset)
The Shimmer Shield is definitely redundant on the Exarch.
From the rules (page 19 under Invulnerable Saves):

If a model has more than one invulnerable save, it can only use one of them – choose which it will use.

So the player would never have a reason to choose the Shimmer Shield unless the player (for some reason) wanted the Exarch to have a higher chance of dying.