Notes:
So here is what they did. Of course they used at the basis a quantum process. In this case radioactive decays rather than the reflection/transmission from a mirror. For radioactive decays again the proper description is that the particle is simultaneously decayed and not-decayed, in other words a superposition of two states. This state would exist till the first observation. In order to be able to observe a radioactive decay one needs some measurement equipment like gamma particle counter. The output thereof was presented to a first observer and subsequently to a second observer. The first observer did not always observe. According to a random scheme the observation was done or not done.
The second observer always looked at the output. Thus, according to the subjective reduction mode, sometimes the second observer observed a collapsed state and sometimes an superposition.
This second observer was then asked to tell if he thought he did observe a collapsed state or a superposition (which collapsed upon his observation).