Many Second Age texts hold that both Gerlant Flamesword and Talor the Laughing Warrior were sons of Arkat and Seleria of Brithos. This is held as deeply heretical, even blasphemous, by both the Rokari and the New Hrestoli.
Talor is widely agreed to have married Ulifla the Sorceress, who aided him on his many campaigns. Together they had twins – a son Mirator, who went on to become the second King of Loskalm, and a daughter Remela, who married Ehilm and founded a ruling dynasty of priest-kings in Fornaor.
In these tales, Talor and Ulifla were murdered by Pakat son of Yemat who had been slain by Arkat many years before. Talar was brought back from the Underworld by Harmast, but Ulifla waited for him in the Court of the Dead.
Harmast doing the full LBQ twice has been around since Cults of Terror, although I think the Guide was the first publication it was made explicit. Early on, Greg wrote that the first Lightbringers Quest was viewed as a failure when Arkat turned to the darkness, and that Harmast went on a second quest to find proper help. He returned with Talor the Laughing Warrior, who later became the first King of Loskalm.
The point of these stories is that Glorantha is broad and deep, and filled with mysteries. We tend to focus on Arkat, and not his sons (who of course couldn’t be his sons, as that would mean that both Seshnela and Loskalm were founded in the Second Age by the heirs of Arkat!), but both Gerlant and Talor have interesting stories in their own right.
For example, Talor’s boon companion was Dakal, a chieftain from the Enjoreli And some versions of the stories have Nralar the Old as the grandson of Talor (not the son of Gerlant) who became king of Seshnela after marrying the daughter of Gerlant. Things were not very clear-cut during the Gbaji Wars or in the early Second Age.