User blog comment:IamJakuhoRaikoben/Chapter 459 & 460: Weakness & The Alighted Pegasus, Review./@comment-3075024-20151101000743/@comment-7418318-20151103170143

@Anon: And who says the chapters would have been in any way different if they came out in seperate weeks?

You just assume that what you call "low quality" is due to Mashima compromising on the quality due to his workload, but that pretty much comes down to a guess.

There has been no word from Mashima himself regarding the issue and I don't really see any difference between these chapters and what we've got in the past.

Besides with the kind of early buffer that Mashima has (see Chaos's post) the workload of a double-chapter would be spread out over so much time that it basically doesn't matter if he does a double-chapter two or three times a year.