If it was a smaller publisher, with less books to publish, that might have been simpler... but Penguin is 
huge, with many authors under its imprint, and print schedules planned months in advance. With all their other commitments in place, putting out both books at once probably wouldn't have been possible, especially considering the quantities needed for a best-selling series like this one. Yeah, we think Jim's a big deal (and for good reason!  

 ) but fast-tracking 
Battle Ground more than 60 days would have likely meant delaying other, even bigger authors' works, and that's something Penguin probably couldn't do, contractually or physically. 
That's my guess, anyway. It might not be completely accurate, since I never worked in publishing, but I did work in bookstores for many, many years, and I remember seeing some lines' publishing schedules worked out months in advance. That might not be the 
only reason, but I'd figure printer limitations and scheduling was part of it.