According to the cover note of the manga volume 10, the second season of Mitsudomoe was announced. The information has already been leaked by animator Kubo Satoshi on August 31st. The new season will start in January 2011.
Sources: Scan of the cover note, Scan2
Updated on September 8th
According to the official website, the second season of Mitsudomoe is eight episodes long.
Source: Mitsudomoe official website
