Of course, all those turned up before the season was split in two, and we only know the first half's writers...
Before it was announced that the series was going to be split in two I'm pretty sure that the splitting the series would have been planned and discussed at length prior to the announcement being made. I doubt that Moffat awoke in Scotland one Sunday towards the end of August and thought "Do you know what would be a great idea? Wow, I'd best announce that today before I change my mind. It's rather fortunate that I'm speaking at a television festival today!"
According to someone who works in the industry and occasionally hears the odd bit about the series there had been serious discussion of splitting series five in a similar manner between Vampires of Venince and Amy's Choice. Apparently they decided against it because they thought it might be seen as suggesting a lack of faith in Matt Smith.
Not, of course that the splitting of the series has any bearing on the writers involved since the all thirteen episodes are being filmed consecutively. Which would presumably mean that the writers would all be working to the same schedule as usual.