Then at that point some adventurous people may have just started trying combining it with every other plant they knew about and every future one they discovered, including plants that they otherwise believed had no effect. They probably would've stumbled upon a lot of interesting combinations.
And if they determined it had a strange ability to seemingly "unlock" "dormant" plants and if they spread that knowledge, maybe some people were trying it with absolutely every non-poisonous plant out there. And since DMT-bearing plants are pretty common (as you mention), multiplied across a whole society it may not have taken very long at all. And it would take just one MAOI plant for them to get the idea that other plants might have this unlocking ability, and just one DMT-rich plant to discover other plants might have similar effects, so maybe they iterated through combinations of MAOIs and serotonergics until they found one they felt had a good balance of maximal power and minimal undesired side effects.
So the bottleneck might be the first instance of someone ever discovering any kind of MAOI plant (this vine or any other). After that barrier is passed, I suspect it might be almost inevitable that something like this particular combination would be found pretty quickly (just blindly speculating, but potentially within a generation or two?).
However, banisteriopsis caapi is orally active on its own: a brew of just the vine will change perception.
So it's an O(n) search to find the ayahuasca vine, then another O(n) to find DMT bearing plants (there are several), which go with it. A typical preparation of the medicine will have a dozen or more plants in it, with two or three recognized as "active" by the standards of Western pharmacology.
So it's plausible to imagine the curanderos adding a bit of this, a bit of that, as guided by the spirits (or intuition, whatever looked good, your choice), then hitting on the combination.
This might sound like I'm being dismissive of what was most likely a directed search for medicinal plants, which isn't my intention. Just simplifying and glossing over that part, to point out that it's a two-stage discovery, and you don't need both plants in a brew to start using one of them.