1. Open the Web browser on the second phone that you want to also use WhatsApp on and go toweb.whatsapp.com.
2. On the mobile browser, it will automatically redirect you to the WhatsApp home page. From the browser options, choose "Request desktop site" (where you find this could vary from browser to browser) and it will then open up the desired Web page with a QR code displayed prominently.
3. On the phone that has WhatsApp installed and running, go to options/settings and choose "WhatsApp Web" (If you are already using WhatsApp Web on any other browser, you will have to log out before proceeding). It will fire up the QR scanner.
4. Scan the QR code displayed on the second phone with the WhatsApp app on the first and it will automatically log in with your account to WhatsApp Web on the second phone. You can now simultaneously use WhatsApp on two phones.
The catch is that you can use this method to use WhatsApp on only one additional device (other than the primary phone on which WhatsApp is installed) and to use on another device, you will have to log out from one.
Also WhatsApp Web isn't a full-feature replacement for the installed version of WhatsApp. You wouldn't get notifications and the interface isn't optimised for mobile use. You might want to switch to landscape to for a fuller view
