Project Nijū (事業二重, Jigyō Nijū) was a scientific endeavor carried out by Shūhen Kōhai and his seven apprentices, with additional help from Bansui Amatsuki and Anko Tayuya, with financial backing from the Collective Vices. It's goal was simple: create the "ultimate Shinigami".
The goal of the experiments where to create the "ultimate Shinigami". It was clear that a Shinigami grew in strength the more they opened up to and familiarized themselves with their zanpakutō partner. Using Modified Soul technology, seven individuals where created, each of which could use not only one, but two zanpakutō. Bansui was in charge of the observations of the subjects, as well as the experimentation carried out on zanpakutō spirits, as outlined in his research findings pertaining to the project; which Jinta Kanō recovered during the foray into Soul Society.
The Tsuinkai (ツイン解, Twin Release) was also documented within the paperwork Jinta recovered, detailing how one perfects the imbalance between both zanpakutō the wielder possesses. How one does this is currently unknown, as Jinta has yet to put the knowledge to practical use.