I too had such a doubt a few months back when I came to know from a friend that "Parallel Space" App from Playstore would help you for such requirement in Android devices. So, I installed Parallel Space in 64Bit per guideline and opened that app, it will auto tick few apps like WhatsApp, Gmail, FB and others.
You have to check or uncheck the required app and install it to use the required one. Once installed it, you can go to the same Parallel Space app again and would open already selected apps within (like WhatsApp or Gmail etc.) to sign up or sign in for the second account from the very same smartphone.
It is also called 'clone' one. There are other few apps similar to Parallel Space but no idea about those apps. However, this app also would sometime annoy you when ads come. Rest everything is good like you can use two Whatsapp, Gmail, FB etc. in the same Android smartphone or Tablet PC etc.