To date, multi-window or split-screen functionality on the Android OS only allows one app to be active at any one time. Thankfully, the good people at Google are reportedly working to improve that feature with the release of Android Q.
The feature is called “Multi-Resume”, and like its namesake, will allow more than two apps to run simultaneously. Without either one of them having to enter into a pause state.
For context, apps on Android currently only allows single instances to run at any single time. In Multi-Resume, this behaviour is abolished; the apps running more akin to applications on a Windows desktop.While some Google apps (like Youtube and GMaps) can already float or use picture-in-picture mode, the information shown on the small floating window can be inadequate. By docking it to the screen, a larger map area in Google Maps, for example, can still shown without it paused waiting for the user to activate it again.
Android Pie already comes with this function, but OEMs need to activate it through an update. App developers just need to include the support in every app. With Android Q, Google plans to roll out the feature system-wide, making it a mandatory behaviour.
(Source: XDA Developers)