There is also the possibility that Black-Kyubi chose to stay with Minato when everyone was released from the seal (perhaps in order to see how the situation looks like before going amok). Right after the release, Oro(or someone :p) states that Madara was resurrected and a 4th Ninja war has started, thus again Kyubi might prefer to be used by Minato (that was revived only for awhile), instead of being used by Madara, the same conclusion to which the White-Kyubi came to. It is a very reasonable action, since Madara can summon a Kyubi whenever he wants, unless it is inside someone else.
Also, it was already shown that the Bijuu can use telepathy to communicate with each other, it is quite likely that it is even stronger between the two halves of the same Bijuu (so the other Kyubi might have already learned what the other one knows).
It is worth noting that when Naruto released Kyubi from the cage and sealing device, the Kyubi still stayed within him, it was not released. What's more, at the last page of the current chapter, we see a glimpse of the Black-Kyubi that is also uncaged.