The Brothers, Irthor and Kidan, are newcomers to the realm of the Divine, on a cosmological scale. The first myths of their genesis originate from the early texts dating to the Age of Stone.
Not much is known about their origins, but it is believed that the Dwarves first started worshipping them in the early days if their own religious renaissance. And then again with additional fervor, with the expansion of the Dwarven Imperium.
It is written that The Brothers originate from Artificium Campestribus, the extra-dimensional plane of Inventions and Arcane Secrets, from where they studied the creation of the universe for eons, before they manifested themselves in divine form on ad' Materia.