The line of Dost and Dast is usually marked to have been founded sometime before 460YM. Whether the father of both is credited with the founding, or the brothers themselves is disputed, but their influence on history can not be refuted.
Dost and Dast were born to a bastard of the Line of Magnus, likely a bastard brother of the Magnus. As twins, they were nigh inseparable, and as children the two were indistinguishable in appearance, yet significantly divergent in personality. Dost had an adventurous and roguish spirit, later in life going on to explore unknown lands and recover ancient artefacts, while Dast was a scholar at heart and preferred to catalogue and orchestrate his studies much closer to home. Both had brilliant minds, surpassing their tutors from a young age and going on to become powerful mages. And both chose to sail away from Magnapur to found their rajdoms. Yet before their left, both had nicknames, likely bestowed the other, that would define their personalities rather well - Dost the Dastardly, and Dast the Dusty.
Dast journeyed less far, heading into the Palaan Sea and establishing a colony between Lukbar and Medhrapur. Though philosophically and literarily minded, Dast sought to found a city of peace and prosperity - likely to avoid interrupting his research. As such, Dastipur has maintained a stance of neutrality for centuries, with the aim of preserving knowledge by avoiding conflict. The Great Library of Dastipur is the foremost magical institution in the known world, and zealously defended by the Shieldbearers of Knowledge (often sarcastically referred to as the Bookbearers). Dast founded the library and contributed significant funds in getting it established, though in the modern day it is an institution that has no strong attachment to the governance of Dastipur.
Dost meanwhile was much more adventurous. For his "grand expedition" he headed along the lightwards coast, further than any Magnapuri sailor had gone before. Eventually, he reached a beautiful tropical peninsula, on the counterroundwards side of what is now the Bay of Badir. This was uncharted land at the time - the furthest counterroundwards any Magnapuri colony had reached was Biltukh, and Dost wanted to go much further. His exploration of the coast is perhaps one of the catalysts for the significant Magnapuri settlement in Luxiterra in the 500s and 600s YM. Dost's eventual settlement - one of the furthest from Magnapur outside the Jisvic Hierarchy, was a masterstroke of integrating his few Magnapuri sailors with the local Xilaq and Barit populace. Not all the Xilaq were peacefully integrated however, and to protect the nascent city, Dost built the Fortress of Dostipur surrounded by the Great Maze, an enchanted labyrinth filled with all manner of traps and creatures to protect his fortress.
The brothers stayed close as they grew older, despite the distance between their cities, and they visited each other often to show off their various projects. Dast married the only daughter of the Raja of Medhrapur, with the dowry being the inheritance of the Rajdom, and he had three daughters, the eldest of whom was Damasta. Dost meanwhile never married but did sire several bastards, many with a Xilaq paramour that he claimed was the nearest he would come to having a wife. The eldest child with this paramour was a son that Dost named after his twin. On a visit to Dastipur, Dast the Younger seduced his cousin Damasta, and the two would meet in secret on subsequent visits between the cities. After a few years, with Dast the Elder considering marrying his daughter off, they eloped, shocking their parents who had thought their closeness strictly platonic.
Though Dast the Elder and his brother did attempt to pressure them into annulling the marriage, Damasta was already pregnant, and soon bore twin boys. The royal line of both the Dostipur and Dastipur Rajdoms has for many generations been able to trace itself back to one or the other of these twins, in most cases both - there have been many more marriages between the two royal families. This is why the family is widely regarded as one entity - the Line of Dost and Dast, as all members of each royal family are descended from both of the Line's progenitors.