Two unrelated plants, out of the tens of thousands in the Amazon, must be combined for either to have any effect, and nothing about the appearance, taste or separate action of either suggests the other. Explanations range from very long systematic experimentation to the answer the traditions give, which is that the plants communicated it. The record is oral and the practice is old.
What would resolve this Probably nothing can settle it. What would help is comparative work on how many other plant pairings are known in Amazonian practice, since a tradition with many such combinations implies a method, while a single one implies an accident or a transmission.
OpenEthnobotany, Amazonian anthropology, ethnopharmacologyLuis Eduardo Luna, Vegetalismo: Shamanism among the Mestizo Population of the Peruvian Amazon