Use APKPure App
Get Split Screen Helper old version APK for Android
Enable long-press for split screen mode
The long-press action to enable split screen mode was removed in Android 9. This app will reenable this feature. Simply long-press on the overview/recents button.
Only tested on software navigation bar. Untested for hardware keys.
This app uses the accessibility service to enable this feature. Due to the sensitive nature of the permissions required, this app is open source.