Yeah this one's a doozy. Mirror is essentially Schrodinger's Thaumaturge, the image (and the thaumaturge!) are both fake and real until some event forces a clear determination. The events that do so are:
- Beginning the Thaumaturge's next turn.
- The Thaumaturge voluntarily moves.
- The Thaumaturge falls unconcious.
- Adept: An enemy within 5ft of the Thaumaturge or the Image damages either (and you can't prevent this, which is why Adept is actually quite a huge downgrade and I don't understand why this isn't a choice rather than forced)
This is a curious case because true seeing doesn't end effects, unlike other counteract checks, it just lets you see through them. But seeing through Mirror would end it...except you can't really do that because as I said, while the rules refer to the Thaumaturge and the Image as separate, you can act from either space and occupy both, you're basically two characters until something forces you to go "ok fine, this one was actually an illusion all the time".
Just for ease of play I'd rule true seeing does nothing here.
Agreed. True Seeing would confirm the truth - that both instances are part of the same entity and that damaging or taking damage from one of them is just as good as the other. It isn't like a Simulacrum, Mislead, or Illusory Creature effect where attacking one of the copies is useful because it damages the actual target, but attacking the other one is less useful since it just damages a rather meaningless distraction.
I'd probably describe the effect as both Thaumaturges looking slightly immaterial and tethered together in some way. Or ask the Thaumaturge player how they think it would look.