iOS 8 includes code that allows for split screen apps
So… just in case there was any doubt left… iOS 8’s SpringBoard has code to run two apps side-by-side. 1/4 size, 1/2 size, or 3/4 size— Steve T-S (@stroughtonsmith) June 9, 2014
I have a terrible feeling this wasn't mentioned at WWDC because this is going to require an iPad with more RAM, so they're saving this for the next generation iPad Air. May be a killer feature to talk about when announcing it.
I hope I'm wrong of course, but I can't think of many other reasons they wouldn't mention it. Regardless, still nice to know it's going to happen.