During the Xiaomi Mi Max 2 launch event, the company had showcased the split-screen functionality available on MIUI 8. On the official Weibo account of MIUI, it has been confirmed that the split-screen function will be arriving first to Xiaomi Mi 5 as trial version as closed beta. Since the trial edition is not yet stable, users may encounter some problem. Users who wish to try the feature can apply for it through the MIUI official forum.
The split-screen function of MIUI 8 is based on the native split-screen function available on Android 7.0 Nougat. The closed beta will not be lasting long and soon the company may begin with open beta that will be provided with a stable ROM featuring the split-screen feature. Currently, only the Chinese ROM will be bringing the split-screen function.
As the name suggests, the split-screen feature will allow users to run two apps in separate windows on the same screen. The feature is expected to be available outside China only after it is successfully rolled out in the home country.
(source)