Yunakiti reports on the 2ch thread where otaku realize that Geisai 12 took place right next to Reitaisai, in East hall 1. Guests there included Yamakan, Ozoku band (Tissue Princess and other Nico Allstars), and Zombies, the ultimate anison dance group.
I kind of feel bad about not knowing about this, but on the other hand it looks like if I went I’d be giving money to Takashi Murakami, and he doesn’t really need any more money than he already has. Not to mention that I honestly believe that Reitaisai is a more significant event of the two in basically every way! Well, except for no yamakan.