Whatsapp and Facebook Messenger are the most important app on our phone. It is now more easy to use this app now. You can use these 2 messengers inside one app, it's called "Disa", and you don't need to switch the app while the messages come. You can see and chat your facebook and Whatsapp friends inside Disa app. The application is coming soon in iPhone too.
The application is available in google play store so there is no need to doubt the security of the app.
You need to download Whatsapp and Facebook plugin through the same application. You can use Disa along with official WhatsApp application but use different numbers.The application is very easy to use. Simple user interface. The application is used by more than one million users and is rated 4.2 stars on play store.