Steven Moffat, Explains....
In DWM Steven Moffat has his own page at the beginning of the magazine called "Production Notes" well in DWM 426 (That's the Cybermen Special and it's why I own it!) Steven goes into explaining the series split and there are a few thing which come to the fore.
1.) Dear Steven even isn't sure even at this time, as to how the series will be numbered in the future, seeing as he's called the series 1, 5, 11, 31 and Fnarg by his own admittance. Fnarg being my personal favourite.
2.) Spoilers or not, so if you don't like possible spoilers skip the first paragraph that Steven has written.
3.) The work load. Steven himself has said (Oh here I go again numbers and Letters.) That A.) That the Split was his Idea. B.) That the work load is exactly the same even with the split and Sherlock in the middle but at least with Sherlock he can delegate. and C.) This is not a cunning plan by the BBC to reduce the amount of Episodes per year.
To Point 3a & b. (Sheees Letters and Numbers, sorry People!) I only have to agree as it is Beeping obvious, because in total there are 14 episodes in the year as there has been for a couple of year now. 13 main episodes and the Christmas Super special edition. So if you want to calm down and come out of the world of panic, then I suggest you read the article. It's funny and you get to see a little of what goes on in Steven Moffat's head, and then you can decide if you want to be him. To end I would like to quote Dad's Army and The HitchHikers Guide to the Galaxy. "DON'T PANIC!" everything will be fine, to quote Ken Deep in Moffat we trust! I know I do, Fnarg.