Install an app called Parallel space from Google play store. Free version has all the features but has ads. You can use whatsapp of first SIM using default WhatsApp app. For the other SIM, open parallel space app and select add WhatsApp. Now setup WhatsApp as usual for 2nd SIM. Now you can use WhatsApp from boths SIMs simultaneously. It's better to buy the pro version for smooth experience (costs INR 150)


Click the WhatsApp icon and you will be asked a “Permission Request”. Just grant it.
After, granting the permission, WhatsApp will start opening and a notice (shown below) will pop up asking you to install an app called Parallel Space – 64Bit Support.


Got to the Google Play store and download and install that app (Parallel Space – 64Bit Support)
After downloading that second app, go back to the first Parallel Space you were using and click the WhatsApp icon again to see magic
There you are, once you click the WhatsApp icon in the Parallel Space app, WhatsApp will open just like how it does every day in your phone

The rest is history, from here you know what to do to. You just have to put the other number you want to use as the second WhatsApp account.
There you have it, you now run two WhatsApp account on one phone.