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Premise

In my story, magic operates through a mana field that can be tapped in to cast spells. Mana comes in two kinds: blue and orange. Flows of mana of either kind are subject to elemental influences: mana streams are naturally found along water or wind streams, trapped into some geological sediments and expelled with lava flows.

Humans, like most living species, are not normally able to perceive the presence of mana (no more than we can feel, say, magnetic fields).

However some druidic society, living in forest areas, has discovered a special berry. When consumed it temporarily modifies the vision, allowing to see flows of blue mana. The way this works is by making blue cones sensitive to blue mana rather than normal blue light, hence dense concentrations of mana appear blue to those who consumed the berry.
They have prospered thank to the use of magic, but never discovered the existence of orange mana, even far into medieval times, long after the existence and usage of magic has been well established, studied and theorized.

On the contrary, a society of nomads living in an arid desert has discovered independently the use of orange mana, by consuming the venom of some snake. The effect on vision is similar but affects orange cones, which among humans only some women possess. The venom also affects green cones but to a lesser extent. Therefore this society has mostly female mages and has grown to be matriarchal as a result. For the purpose of the question, let's say that around 30% of women are born with this additional cone.

As some point, descendants of these nomads became sedentary and established cities in the desert. They also theorized magic and have a good grasp of it, though not aware of the advances of the druidic societies.

For my story, it is very important that at some point in history, a cultural clash between these two civilizations be rooted in their completely different handling of magic.

Last but not least, both societies have been in contact with dragons, which are among the rare creatures naturally sensitive to mana. Although this vary with breed, most dragons can see and manipulate both kinds of mana. They are far more intelligent than humans but do not normally bother with learning how to communicate with their delicious preys.

Question

What is simplest possible explanation (i.e. involves the smallest amount of additional elements) that each society would have independently discovered and studied only one sort of magic, without ever accidentally stumbling upon the other kind?

Some elements I'd like to emphasis:

• In this universe, magic can in principle be learned and used by (nearly) anyone, (nearly) anywhere. (i.e. your answer can't be "blue mana is only available in region X, orange mana in region Y"). In the future the two societies will reach peace and the use of both types of mana will spread.


• While your answer may involve some genetic predispositions, no human is born with magic abilities, as mana can not be detected by the human body alone.


• It's perfectly OK that (some) magical creatures would be sensitive to both mana types, but no human group has ever discovered both, some have discovered neither.


• Although both the berry and the snake are endemic of the said forest and desert, respectively, they are many other species that use either kind of mana as part of their metabolism. Some common minerals tend to trap high concentration of mana and are not specific to a given region. neither the snake nor the berry are necessary conditions for the discovery.


• The nomads and druids descendants have lived fairly separately before the above-mentioned clash. They know of each others through tales of itinerant merchants, but they never suspected that their magics were so deeply different.

More Background Info

The following is not necessary to answer the question, but provides background for the curious.

• Mana in itself is actually colorless, the names only refer to their effect on human vision.


• In my world, the discovery of the berry is the root of druidism, as druids maintained their privileges in early human groups by consuming the berry and using mana to become valuable members of the group, including communicating with other beasts that consume the berry.


• The reason the berry has this effect is a dissemination strategy: the berry traps mana so that crows, who consume the berry in their normal diet, can spot the berry easily and spread seeds throughout the forest.


• The reason the snake's venom has this effect is part of their mating behavior: males harbour beautiful patterns obtained by fixing orange mana to their scales. The pattern only reveals to females that they have bitten as part of the courtship ritual, but not to potential predators, insensitive to orange mana.

It's simple. The snake that induces the ability to see the orange mana only exists in the land of the nomads. The berries that induces the ability to see blue mana only exists in the land of the civilization.

Though, eventually, there will be wanderers that go into these lands and discover the oranga mana inducing snake or the blue mana inducing berry. But, the nomads had built up through many generations a tolerance to the snake venom. So, if a wanderer is to consume the venom, she might be able to see orange mana, but only for a little while before collapsing and dying. Same thing with the berries, they're poisonous, and the blue mana users have through generations become tolerant to it, but the nomads haven't.

Also, just to take note of; venom only affects people when injected into the blood. Nothing will happen if you drink venom. Though poison on the other hand, affects one both through blood injection and consumption. Just have that in mind when writing on.

Good luck, I really like the concept.

Simple. Berries are particles. Snake venom is a fluid, so it has wave like behaviors. I swear to god, holy wars have been raise in the science class room whether light is a particle or a wave, despite the best efforts of the teacher to teach that they're two sides of the same coin. Why should your manna be any different?

Your "Blue cones" in the eye are sensitive to ultraviolet light, "orange cones" are sensitive to infrared. They are just outside are both just outside the spectrum of frequencies visible to human eye, but on opposite ends of the spectrum. So ability to see one of them has nothing to do with ability seeing the other.

In fact, you can make it so that ability to see ultraviolet could come at the cost of ability to see some deeper reds that are visible to to normal humans. And vice versa, seeing infrared costs you ability to see blue. Finally, you could make it that taking both blue berry and snake venom will pull visual range in opposite directions, leaving a person blind.

Also: some birds can see ultraviolet; some snakes can see infrared.

Edit: Biology.SE tells us that humans have some biologic mechanisms that could let them see either ultraviolet or infrared, but they are different mechanisms.

So your blue berries weaken or destroy the eye lens (creating aphakia), or make it transparent to UV rays. The venom mutates eye proteins and shifts the range that "red" eye cone can detect.

Edit 2: If you want somebody to do both kinds of magic, you can assume that ability to see UV and IR can happen in the same person, i.e. the two different mechanisms do not affect each other (my statement above against this is a hypothesis, not a fact).

Simple. In order to learn how to use something you need to see it to study it.

The Druids have only ever seen blue mana and been able to manipulate it while under the effects of the berry. This has allowed them to craft spells around using blue and its limitations.

The nomads likewise have only ever seen orange due to the lack of the berry and have only learned the properties of orange mana.

Skilled practitioners could likely have learned to use magic by feel after using it so frequently with the berry/venom much in the same way as you can navigate your house in the dark. It is something that anyone could do with study but it is night impossible without the "training wheels" of the "sight".

Given OP's updates in comments (which they should edit into their question, as it changes everything), there is no answer that we can provide except by guessing and speculation.

Example:

OP says there are other objects as well in each area that also provide access to both forms of mana. So why haven't they taken advantage of those?

A: The orange berry in the forest is shaped like your peoples' symbol for "fart cloud", so nobody has ever tried to eat it.

Technically, that answer is just as correct as any other. And if OP is writing a humorous story, then I ask you to please use this answer, and combine it with the following: The nomads do not try to use the blue snake because it commonly curls up to sleep in the pattern of their language symbol for 'one who sneezes just before everyone runs at the hunting prey' and nobody wants to be that guy.