Now after some time the same WhatsApp account can be logged in on different devices. Soon the same WhatsApp account can be logged in up to five devices at the same time.
It is called companion mode of WhatsApp. WhatsApp had been working on this feature for a long time. Companion mode was already available for PCs.
Now this mode is also available for phones. This mode is also called link with your phone feature.
It allows you to log in to WhatsApp on your secondary devices and use it. This feature is currently in beta testing.