This is going to be a short thread, but it seems that with the new chapter there are still people who don't understand how or why Minato has control over Kyūbi, so I wanted to explain: when a user of the Dead Demon technique seals himself within the Shinigami, he and the victim (the person whom he sealed) exist within the demon's stomach. They don't "die" or remain in stasis such as actual death. That being said, while Minato has been away for fifteen/sixteen years, he has NOT been dead for that time. He's been alive in the Shinigami's stomach with fifteen to sixteen years to combat this creature. He's basically had the entire series and then some to subjugate Kyūbi, and as the creator of the seal, he obviously knows how to manipulate it to maximum effect: recall how restored Naruto's seal and then amplify that several times over (I'm willing to say a hundred times considering he's a Fuinjutsu genius, but that's admittedly too superlative).
As to why there are "two" Kyūbi, the answer is simple here as well: Bijū have long since been defined as creatures of pure chakra. By splitting the "yin and yang halves" of Kyūbi's chakra, Minato quite literally split the fox into two beings. However, if rejoined, this duality would cease to exist, at least that's my theory; it remain depending on how Kishi envisioned it.
As to why there are "two" Kyūbi, the answer is simple here as well: Bijū have long since been defined as creatures of pure chakra. By splitting the "yin and yang halves" of Kyūbi's chakra, Minato quite literally split the fox into two beings. However, if rejoined, this duality would cease to exist, at least that's my theory; it remain depending on how Kishi envisioned it.