Google said that it will now allow developers to keep apps in multiple windows in a resumed state simultaneously in Android 9 Pie. Developers just need to add the required meta-data to their app's manifest to opt-in. Multi-resume is part of a series of developments to make Android more friendly to foldable screens. Although this feature cannot be tested in current devices, OEMs are expected to push out OS updates that will enable developers to test out this functionality for their apps soon.
https://www.notebookcheck.net/Android-will-soon-offer-multi-resume-functionality.360561.0.html