Lar Gand and Halk Kar had been explorers of space prior to their arrival on Krypton-One and -Two, when they met the scientists Jor-El and Jor-L respectively. Sent back into space by the scientists prior to the destruction of both Kryptons, instead of ending up on their native Daxam and Thoron they arrived on Earths-One and -Two to encounter the sons of their Kryptonian colleagues, Superboy and Superman, both of whom thought that this pair were long-lost brothers. Neither Lar nor Halk could neither confirm nor deny whether this was true due to temporary amnesia they suffered while in stasis due to their journey. While Kar was able return to Thoron unscathed by the meeting, Lar suffered lead poisoning which meant he had to exist in the Phantom Zone for a thousand years before the Legion of Super-Heroes cured his poisoning...leading to Lar as Mon-el joining their organization. That is, until his descendent while trying to protect him for danger ended up exposing him to greater danger when Mon-El's poison antidote had expired, and so did he!
As a result, this descendent Eltro Gand assumed the alter ego of Mon-El and then exchanged his life with that of his ancestor's, resulting in his brainwaves being transferred to Lar. This led to a new composite Mon-El who found a counterpart in Doctor Kent Nelson, who had himself taken the place of his late father archeologist Sven Nelson and had the composite memories of Nabu and his own. Lar/Eltro knew of ancient cultures having observed them while non corporeal in the Phantom Zone, Kent/Nabu had memories from the days the Lord of Order reigned supreme. Both Kent and Eltro possessed super-strength, invulnerability, swift flight and potent powers of perception. While Eltro had inherited his powers genetically as a Daxamite while living under Earth-One's sun, Kent gained his power training under Nabu the Wise from the planet Celia. As a Legionnaire and All-Star, Gand and Kent worked beside the greatest superheroes of their adopted Earths in Superboy and Superman as Marvel Lad/Mon-el and Doctor Fate both in times of peace and war. Eventually, both heroes created special rings harnessing the power of anti-gravity and expanded their minds in various fields of science as well.
As for Nabu, his doppelgänger was Nommo, each having been long-lived legends protecting twin Africa's from millennia ago into the modern era with Nommo as Doctor Mist and Nabu as the first Doctor Fate. Another Nabu and Doctor Fate arrived from the post-Crisis composite Earth to protect Earth-Two in the Asgardian dimension created by Hitler's madness and the Spectre's dormant power. This entity along with Sven Nelson were the counterparts of Earth-One's Doctor Chaos and Burt Belker, the archeologist who like Sven first encountered The Lords of Order and Chaos. Chaos himself had like the second Nabu originated from another dimensional plain and appeared once, with Doctor Chaos's helmet being dispatched into the abyss after a confrontation with Superboy, and with the Doctor Fate of Asgard returning to his native dimension after a decisive battle involving the Justice Society of America and the forces of Hitler had concluded. Meanwhile, archeologist Bert Belker would live out a normal life thereafter, while Sven would unfortunately perish after having located Nabu.