Thor lets go of Tony's hand and cautiously approaches the mirror, waiting and expecting his own image to replace what he's sure is a figment of his imagination.

It doesn't happen.

As soon as he's within arm's reach of the reflection, he reaches out and taps the mirror right where the other Tony's forehead still stands, fully expecting the illusion to dissolve right then and there as so many of Loki's had when he conjured them as a child.