WhatsApp may allow single account sign-in on multiple devices: Details here

The multi-device sign-in feature would allow users to use single WhatsApp account on multiple devices simultaneously

WhatsApp is testing multi-device authentication, which shall allow users to sign-in from the same account on multiple devices. Currently, user can authenticate single device by means of mobile number verification. As soon as you change the device and sign-in on another device, WhatsApp automatically logs you out from all other devices. The multi-device sign-in feature, which is currently tested internally by WhatsApp, would allow users to sign-in on the same account from different devices.

“From now on WhatsApp is internally starting some important tests for the multi device feature. It's not available yet and there isn't a release date: it could