My guess is they already knew about the MAOI to at least some degree and noticed that the effects of other plants were amplified when combining them. So when they found the DMT plant, someone figured it'd be worth seeing what happens when they combine them.

If it's true that the DMT plant has no psychoactive effects when taken alone, maybe someone had already had a routine of taking the MAOI and just eating most plants they came across, to see what would happen.