It works:


The mirror is 70% transparent. You may try different transparencies using the source images. With Image Magick installed, compositing is a simple command: In reality, the scene behind the mirror would be quite dim, and the transparency much lower, maybe 15%. That makes it harder to look through from the front and accommodates the observer behind to the little amount of passing light.

The mirror leans 1° away from us. Therefore the mirrored horizon is 2° below the one in direct view.