We will use an application called 2lines for whatsapp
[NEEDS ROOT]
You can download it from HERE (playstore) or HERE (Direct APK Link)
Usage:
Open 2lines, it will automatically save your current line
Then tap "Add a new line for whatsapp" it will create a new profile
Now you will add the new number like first time you installed whatsapp
This way you can SWITCH between the numbers whenever you want
GO HERE if you want to use multiple numbers simultaneously WITHOUT ROOT (New!)
If you installed whatsapp+ you need to uninstall it, install original whatsapp, re install whatsapp plus
