Whenever a Solosis reaches a certain age, it begins to undergo mitosis, which typically leads to the birth of two new Solosis. A few of these pokemon, however, fear that dividing their power shall make them too weak to triumph and attempt to halt this process, while others, especially in captivity, find simply that there is no room to create a twin. Whenever this happens, a Solosis undergoes an incomplete division, with two closely related minds and an altered cell structure to accommodate them inhabiting the same body, and a Duosion is born.

Duosion do not quarrel like multi-headed pokemon, for the one thing which unites these heads by definition is the desire not to endure the weakness which comes from a full cell division. But the two minds have already divided from their predecessor, and do not have clearly delineated influence over respective parts of the body. When a Duosion thinks in unison with itself, it is a powerful pokemon, its psychic attacks second to none among pokemon yet to fully evolve. But far more often its minds clash and the Duosion becomes frozen by conflicting instructions, or its body attempts to do opposite things at once, with hilariously incompetent results.

Duosion, for all their power, are an incomplete pokemon, and can be meaningfully compared to Metapod or Kakuna in the changes taking place within. Their minds and bodies, not quite separated, attempt to merge back into themselves, but what is done can not be undone. When a Duosion's two minds finally manage to reunite, the result is not another Solosis – even a giant one, but something far more powerful.

Reuniclus do not undergo cell division, for they are born when the halves of the Duosion reunite, and they are strong and traumatized enough to prevent mitosis outright.