The new feature, which is still in beta, is accessible to everyone using the latest version of WhatsApp. To access the multi-device feature, open WhatsApp, navigate to Settings, tap on Linked Devices and you’ll see a new ‘Multi-Device Beta’ option.
It’s worth noting that once you join WhatsApp Beta, you will be unlinked from all devices and have to re-link. Once linked, the feature will work similarly to how it always has, with the exception that you will be able to send/receive messages regardless of whether your primary smartphone is online. Messages cannot be deleted from a linked device. For now, that functionality remains exclusive to the primary phone connected to WhatsApp.
Also worth noting is that you can’t message or call users who have an older version of WhatsApp on their phone from the connected devices, and you can only have one primary phone connected to your WhatsApp account at a time.
Your primary device also needs to be accessed actively. If you don’t use the primary device connected to WhatsApp for 14 days, all the linked devices will automatically get unlinked.