Unlike most of its competitors, WhatsApp does not allow you to use the same account on multiple devices. This defect is inherent in the operation of the application which does not store messages on servers unlike Skype or Facebook Messenger. All data is saved and encrypted on the user’s smartphone.
As a result, WhatsApp has since struggled to implement cross-platform usage. According to the WaBetaInfo site, a feature under development will soon link the same account to several devices. A new notification was indeed identified in the last beta of WhatsApp. It allows the user to receive a message indicating a change in encryption key when the current number is added to another device.
This new feature clearly indicates that it will soon be possible to configure the same number on one or more other devices without the account being blocked on the first. We await to see how it will work exactly. It seems unlikely that WhatsApp will adopt the backup of messages in the Cloud to allow you to find them regardless of the device on which you are connecting.
Instead, the app could locally synchronize data in real-time on different smartphones. For now, the only way to take advantage of cross-platform use is to go through WhatsApp Web which can be used from a browser or via a dedicated application on a computer.
You have to scan a QR code with your smartphone and keep it constantly connected to the Internet to be able to receive and send messages on the second device. This option is certainly practical, but not very ergonomic, especially when you use it on another smartphone, the interface is not optimized for comfortable use.