Why would two humanoid species be physically unable to touch each other?

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.

posted over 6 years ago

Secespitus‭

-2 reputation 6 24 -1 0

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.

posted over 6 years ago

BobJarvis‭

1 reputation 0 7 0 0

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.

posted over 6 years ago

_X_‭

1 reputation 0 24 0 0