Yeah. If the psilocybin effect was merely a placebo effect, that effect would probably wear off after a while, and it would stop working. So this is detectable,

Yeah. If the psilocybin effect was merely a placebo effect, that effect would probably wear off after a while, and it would stop working. So this is detectable, though not in a short term trial. The proposal with using both psilocybin and LSD is based on this assumption: If both substances yield a similar outcome, it is more likely that they both don't work rather than that they both do work (to a similar degree). Presumably the prior probability that two hallucinogens treat depression is rather low. Because most substances don't treat any disorders, let alone depression in particular.