I actually assumed that the sources behind the rumour were actually referring to the new App Extensions framework-- whereby an application can offer up certain types if "plugin" to other applications-- that will indeed be introduced in iOS 8.
However, it appears that the rumour department of 9to5mac are persistent with their story. They cite apparent evidence in the form of a de-compiled section of code from the beta development kit which includes references to calls that apparently are designed to support a "multiple app display layout" of some sort.
- the de-complied code presented isn't exactly a smoking gun; it's not terribly clear exactly under what circumstances this code is intended to be used;
- given that this year's update is highly developer-focussed, and that a split-screen feature could have significant implications for developers, it's an odd feature to "sneak in" outside of the developer conference-- Apple have now spilt the beans on various major new features to the OS, so it seems slightly unnecessary for them to be so secretive over this specific feature if it is really in the pipeline any time soon;
- with the App Extensions framework, Apple have clearly put some thought into how they want app-app interaction to work; it seems a little clunky for them to introduce a separate feature that simply lets two arbitrary apps sit side by side;
- on the other hand, there is good evidence that Apple are planning to introduce devices with more diverse screen sizes, and a larger-sized device would make a split screen function a little more viable.