Here’s how you can add up to 4 companion Android smartphones or iPhones to your WhatsApp account. Firstly download or update WhatsApp from Google Play Store or Apple Store.
Open WhatsApp on your companion device and click on "Agree and Continue". Enter your country code and phone number, then click on the three dots in the top right corner and select "Link a new device".
On your primary phone, open WhatsApp and click on the three dots in the top right corner. Select "Linked devices" and click on "Link a new device".
Authenticate and scan the QR code that appears on your companion device. This will enable you to access your WhatsApp chats on your secondary device.