Although the Tropius can grow from fruit seeds, Cherubi are the only fruits which have gained the intelligence needed to be considered a pokemon, a fact which unfortunately has made them no less delicious or nutritious. Their parents, the Cherrim, must rely on other pokemon to carry their seeds away from their groves in their digestive system to avoid overpopulation, and those seeds are contained in their Cherubi children. But once the seeds sprout into new Cherubi, they must survive long enough away from their parent to evolve, and it is this paradox which has driven the evolution of Cherubi's second head.
The larger of Cherubi's heads is properly speaking the main pokemon. The second head is a bud, not a full-fledged partner akin to a Doduo's; its purpose, so far as the main head is concerned, is to provide bait and dissuade predators from eating the main head, for it is stuffed with valuable nutrients which bring pokemon closer to evolution. Once the head is eaten, within a week a new second head will begin to grow.
If the second head goes too long without being eaten, it will grow larger and larger, and threaten to challenge the main head for dominance. When it grows large enough to become threatened, the elder attempts to eat the younger and win the nutrients of evolution for themselves, and the younger strives to steal their connecting vine. If successful, it will become the new Cherubi and sprout second heads of its own; its brother, shorn of everything but its center, becomes helpless prey for wild pokemon. These struggles can continue for many generations until a great or lucky Cherubi devours enough of its heads to evolve into Cherrim.