Posters that are the size of two B1 are technically B0, but they are not called B0's in Japan. They are either called two-sheets (tu shiito) or king size (kingu saisu).
If you are interested in more Japanese posters and also general information on them, you can check out my website www.posteroid.com
Plus I have a few videos on Japanese posters on youtube, here are two or them:
Posters of Japanese horror movies:
Posters of international horror movies
I really like your poster videos . How do you do them, I'd like to do something like that with mine. gonna check your web site now.