the multi-device WhatsApp feature has been spotted on the 18.104.22.168 beta update. The feature will be available on both WhatsApp app as well as WhatsApp Web.
Multi device with WhatsApp Web will enable users to use their WhatsApp without their main phone connected to the Internet. As for the app, users will be able to connect up to 4 different devices to their main WhatsApp account. WABetaInfo notes that this limit might change in future.