After Instagram, Facebook is rolling out Multiple Account support to Messenger app. It will allow users to login multiple accounts at a time, you just have to switch to different account to use your other account at same time.
Facebook says that it has added the feature for those who “share their phones and tablets with members of their family and friends.” Further, Facebook quoted, “All over the world, hundreds of millions of people share their phones and tablets with members of their family and friends. At Messenger, we want to make communication simpler, more seamless and secure, so we are happy that we have now launched a feature on Android that enables multiple people to log in and use Messenger from a single, shared device.
The beauty of this new feature is that your messages stay completely private. Anyone else who shares the device will just see the number of notifications of messages that you have, but they will never be able to see the content of your messages.”-Facebook
You can get this features on the latest version of Facebook Messenger app. You can try to manually update the app by opening the Google Play Store, going to My apps and games, and checking your pending updates.