The Never-ending Long Nights

This is a planet of locked rotation and half fire half ice.
One side is the eternal daylight of extreme torridity, another side is the eternal darkness of the bitter cold, and wrapped in the middle is the blessed habitable zone. Residents in the three different zones are having completely segregated lives.
Until the day they were called up to fight the potential disaster of cosmic rays, they came to know the existence of each other for the first time.
Will they choose to unite? or will they conceive their separate evil intentions?