WhatsApp companion mode works in a similar manner as the multi-device mode. If you have received the feature, you will be able to see a new ‘Link a device’ option while setting the messaging up. Once you click on it, you’ll be able to see a QR code that you will have to scan from your primary device to use your WhatsApp account on a secondary smartphone.
As the feature is currently in beta phase, it is not available to a limited number of users. It is expected that the feature will be available to more users in the coming weeks.