Well it's possible that he wasn't aware that he had the power yet, since he passed out.That and now Kakashi was recon as having MS for 15~ years instead of awakening it during the time skip.
Perhaps it's impossible for only one of the eyes to gain Mangekyou, either that or Obito and Kakashi getting MS at the same time was just supposed to be a deep and meaningful coincidence.
I wonder if Madara orchestrated this to some extent using his authority as Mizukage. And on another note, I wonder if this event will soon lead to Obito becoming the true Mizukage, since Kisame recognized him a while back. But I guess we still need to figure out how Tobi and/or Madara were able to do all the things they've done at the times they've done them.The Kiri ninjas also mention the Bloody Mist, which means that Madara must already have been in control of their village.