Board Thread:Support Requests - Designing Your Wikia/@comment-25616959-20150817180143/@comment-4731443-20150818230444

The crucial difference I can see in the two wikis' CSS is that on BB1, the body tag has the class "background-dynamic", which BB2 lacks. As far as I can see, that class is what adds the splitting functionality. No idea why it's missing, though.

Perhaps you could try uploading an image over 2000-pixel wide, make sure the "no split" option is off, then switch back to your actual image?

If that doesn't work, copying the "background-dynamic" class and forcing it onto the body rules in your custom CSS would probably work, but I would definitely recommend asking the Staff before exploring that option.