Why would two humanoid species be physically unable to touch each other?
I need a reason for two antagonistic humanoid lifeforms living on the same planet to be completely incapable of direct physical contact.
I don't mean they have a strong aversion to it, I mean they literally should be unable to on a fundamental, physical level.
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3 answers
The two humanoid lifeforms are two mental states inhabiting the same bodies. The switch is triggered, for instance, by the day/night cycle, or it follows a seasonal pattern. All the planet is simultaneously in the same mental state, so that they can never touch a body in which the mind is in the "other" mental state.
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Magnets
For some reason your aliens developed a strong magnetic field that they are constantly emitting.
Your humanoids use this to attract prey, either by forcing them directly through the magnetic field influencing the prey's body or by for example foiling their orientation. The prey tries to walk or fly around normally and suddenly their organs are telling them to go straight to their right, into the arms of the waiting humanoids. Look at birds and their Magnetoreception.
Because they both evolved this mechanism they now repel each other.
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They both live in swampy environments, each exudes a delightful-smelling musky lubricant that allows them to slither through their environment and protects them from contact with the potentially nasty things that live there - and each species slime is both disgustingly stenchy and a deadly toxin to the other species. Contact = mutual death.
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