In a new discovery found in an AOSP framework commit, Google references the term "multiwindows" numerous times. While details on the subject are scarce, we now have some proof that Google is working on some sort of multi-window feature.
Google, it’s high time for split screen to come to stock Android
Multi window is already on a number of OEM’s devices, but not on stock Android. Even Apple is bringing it to iOS. As smartphones grow larger and more powerful, there's little excuse left for Google not to bring this feature to stock Android.