Open WhatsApp on the second phone
Tap Agree and Continue
On the phone number screen, tap the three-dot menu or the option for linking a device
Select Link to existing account or Use as companion device
On your primary phone, open WhatsApp
Go to Settings
Tap Linked Devices
Tap Link a Device
Scan the QR code shown on the second phone
Wait for the chats to sync
Use both phones with the same WhatsApp account