I wonder too, but I think if we think about this carefully we can note that the plants really grow best in very different climates. This means that to the extent they were combined, it was only by people who had access to one or the other via trade. I think it's likely that until the 20th century, many more remote parts of the jungle and mountains alike did not see much or any trade at all. Areas closer to urban centers of course could have engaged in all kinds of trade going back centuries or more. Medicinal plants were likely highly valued and traded successfully over substantial distances, and any curandero/as working closer to urban centers would have access to a lot more possible admixture plants.
One of the huge problems when talking about "indigenous" things, as a category, is that in most of South America, those lines have always been blurred since the conquest of the West. This is in contrast to the colonialism of North America in which there was much more segregation between colonist and native. Ancient traditions involving medicinal plants, which were already extremely diverse from tribe to tribe (so to speak), undoubtedly evolved in the vicinity of urban centers to accommodate a wider range of customers. This led to an evolution and "Westernization" of a great many so-called indigenous traditions that was going on long before the growing numbers of tourists from the U.S. and Europe started showing up, looking for a "spiritual healing experience".
Particularly frustrating for those who like to wonder as I do, almost all the traditions have been passed down orally, and it seems all but impossible to even scratch the surface of knowledge accumulated across the continent over recent centuries to say nothing of what there was before the White Man came.
