In c.2262, Exar and his Andorian wife Shona voluntarily exiled themselves to the uninhabited neighbor planet, Fornax II. Shortly after relocating the couple had a son, who shortly after birth, developed physiological trauma which was related to his mixed genetic heritage.
With his son facing certain death, Exar launched himself into a massive program of research to determine if his son's life could be spared. As he searched he stumbled upon a scientific paper, written by Lieutenant Commander Spock, which detailed the Alfacite Energy Phenomenon and "hybrid twinning" that he witnessed on Alfa 177. Using Spock's paper as a basis, Exar began developing a genetic duplicator which would be able to duplicate the same effect and split his son into separate beings, one with physiology similar to Exar's species, the other with Andorian physiology.
By early 2268, Exar had successfully tested his duplicator and his son had been split into two separate beings. Unfortunately, Exar realized that he was dying of an incurable disease but desperately wanted to preserve his work so that one day someone would be able to complete his research. To this end, Exar compiled all his research onto a sealed tape and made a request to Starfleet Command for someone to collect the disk and transport it to a secure facility.
Shortly after, the USS Enterprise arrived in orbit and Captain James T. Kirk and Commander Spock beamed down to collect the secure tape off Exar. However, Exar realized that he opportunity to further his research because of Spock's Human/Vulcan physiology and so forced him to enter the genetic duplicator at phaser point. The process was a success and two Spocks were created, and they returned to the Enterprise with the tape and departed, without informing his crewmates of what occurred on the surface.
A day after the Enterprise departed Fornax II Exar fell into a deep coma and eventually died a week later. Exar's tape made it safely to Starbase 10, and Spock was eventually able to reintegrate himself using a similar device to the one that divided him. (TOS - The New Voyages short story: "Ni Var")