The first link is from my post. It will give you an overview of the way I did it. The second link includes a video and shows the specific files to move.
My instructions, I already had Whatsapp running on the iPhone hence the directions to delete it on the phone. I used my Google number and it's ability to receive the text on the iPad so my directions differ slightly from the video. The secret is the two devices (your phone and pad must use different phone numbers as only one device using a given number can be logged in at the same time. (You can not have your iPhone messages repeat to the iPad. )
https://forums.macrumors.com/threads/1690525/
http://www.redmondpie.com/install-whatsapp-on-ipad-and-ipod-touch-without-jailbreak-video/
It has been some time since I did this but if you have any problems come back and I will attempt to walk you through it.