Gigantomakhia is Kentarou Miura's new 6-part miniseries, set in a world 100 million years in the future.
The title is a Greek phrase meaning Battle of Giants. It refers to the mythical war between the gods of Olympus and rebelling giants. For a much more rich exploration of Gigantomachia across the ages, check here.
According to an ad in the Nov. 8 issue of Young Animal, Gigantomakhia will run for 6 issues, likely ending in February 2014, with Berserk set to resume afterward.
The first episode will be 44 pages, with one color page.
We'll know a lot more about the series once we get closer to its release, so stay tuned here!
UPDATES:
Walter said:Confirmation came this morning from 2ch that Gigantomakhia will begin its 6-issue run in the Nov. 22 issue of Young Animal, which makes the release schedule the following:
Issue 1: Nov. 22
Issue 2: Dec. 13
Issue 3: Dec. 27
Issue 4: Jan. 10
Issue 5: Jan. 24
Issue 6: Feb. 14 <3
With Berserk to resume afterward, possibly the Feb. 28 issue.

Cool timing btw, I was actually just about to rewatch MoS. But man! I cant describe how psyched I am for Gigantomakhia, just the title itself sounds so powerful (love Walters japanese pronounciation there
)
). To answer the other part of your question, I don't think that manga authors do this sort of thing very often, but working on two series at once isn't unheard of. Gigantomakhia is an interesting case to me due to its length.