In the most recent naruto chapter,
Many people dont understand how this happens, but I get it
Remember when the 3rd hokage sealed orichimarus arms? He said that the caster and the target of the shiki fujin fight for eternity in the reapers belly.
When minato sealed half of the kyubis chakra inside himself, he was essentially its jinchiriki
The biju are chakra monsters, so when you split its chakra in two, you split the beast in two. I think that most likely, minato has its own kurama. Minato was able to practice going biju mode for a decade in the repears belly
SECOND THEORY, minato could go biju mode because he only has kuramas chakra but none of its will