As revealed by WABetaInfo, the beta version of the application indicates the possibility of registering a device used as a “tracking”, by scanning the code using the main phone on which WhatsApp is used.
In earlier versions of the beta, it was discovered that devices could get the ability to sync recent messages, despite not having end-to-end encryption. There is also a guide with instructions for users on how to use WhatsApp on another device.
All of the above is clear evidence that the mentioned function is in preparation, which will enable the use of WhatsApp accounts and conversations on multiple devices. All features are revealed in the Android version of the app. It is unknown at this time whether the option will exist for the iOS version of WhatsApp when it becomes officially available.
WhatsApp technically supports the use of accounts on multiple devices, via the Linked Devices feature, but it currently only supports computers as secondary devices. The feature is available for both iOS and Android.