Since I have LineageOS 16.0 (Android Pie) I use more and more the “split screen”, holding the square button to have two app almost running in parallel.
As described here:
However, the split-screen functionality, as was introduced in the past, continues to suffer from one grave limitation. While you can open two apps in split-screen view and better utilize your evergrowing screen real estate, you cannot run the two apps simultaneously. When two apps are opened in split-screen, only one app is retained as the active app at a time while the other app is paused.
This is really annoying when you start playing a YouTube video on one screen and it freezes when focus is lost.
Apparently, a new feature will come for Android Q: multi-resume, allowing two apps in parallel.
Where it is not clear to me is that could already be available in Android Pie if the OEM allows it …
Developers can opt-in to this behavior by simply including this meta-data in the app manifest.
*meta-data android:name="android.allow_multiple_resumed_activities" android:value="true" *
Currently there are no devices to test out this behavior but OEMs are expected to push out updates to their devices to allow developers to test their apps.
To conclude, a little trick that I just discovered: if the app refuse to go to split screen it is possible to force it in developper settings.