WhatsApp has a small Vulnerability which allows you to run the same WhatsApp account on two phones provided they both have the same MAC address. Follow the steps below
Uninstall WhatsApp from your phone or device. You will install it again at end of the method, but you will need to configure it differently, and the easiest way to make sure you’ll be able to do that correctly is to completely uninstall it now.
Find your targets phones (MAC) address.
A MAC address is a unique identifier assigned to your phone or other device that essentially deserves as its online identity. Find and write down your targets MAC address. It takes the form of six pairs of letters or numbers, separated by colons (01:23:45:67:89ab). On an Android, find this settings ->About phone ->Status -> Wi-Fi MAC address.
Find your MAC address
Find your MAC address using the instructions above. Make sure to save it somewhere secure, as you’ll want to change your device back to that address once you are done.
Change (spoof) your phone MAC address to that of your target.
This will allow your phone to pass as your targets, letting you convince WhatsApp that you’re your target when you log in.
-> On Android, Install BusyBox. And see how to spoof MAC address section from main list to get details of spoofing MAC address.
Install and configure WhatsApp on your phone. Enter your targets phone number to set up WhatsApp rather than your own. This will allow you to receive their messages and send messages from their account.
Get the confirmation code from your targets phone. This code will be sent to their phone number as part of the WhatsApp configuration process. Access their phone one last time to get that verification code.
Enter it into WhatsApp on your phone when prompted, and you are all ready to go.
-> If you do not want your target to find out about your spoofing, make sure to delete the confirmation code from their phone before they can see it.
When you no longer need to access your targets WhatsApp account, change your MAC address back to where it was.
Repeat the instructions to change it, but this time change it to your original address rather than your targets. This will restore your phone to how it was originally, which will ensure that there are no future problems with Internet usage or connectivity.