Visionox performed the tests on a display module that included the AMOLED panel, a touch layer and a thin circular polarizer layer. (oled-info.com)
The patent also describes use of polarizers in order to minimize reflections from the two screens onto each other, which is a clever addition to solve that problem should Apple actually look to build this as a real device. (theverge.com)
You can see how polarizers work by putting two of them together. (scientificamerican.com)