Two stage screens polarize light and with the possibility of reducing the visibility of the instructor/safety pilot so their use was prohibited in the UK.
Never heard of that one before. The two stage amber screens were used in Australia during and after the war and were universal in flying schools before gradually being replaced by other sorts of simulation. I am not sure what is meant by polarising light though. After all the instructor didn't wear the blue goggles and he could easily see though the amber screens that were clipped to the inside of the front and side windows.