Tools:
1. Original Whatsapp - Download HERE
2. Your iPhone
3. Your iPad Mini or iPad
4. iTools you can get it from HERE for Mac or HERE for Windows
5. Some guts!


*This tutorial based on Mac, Win might have differ way to navigate

Set A - Registering the WhatsApp with phone number
1. First install original Whatsapp on the iPhone
2. Go thru the authentic process. (Register your phone number and etc.)
3. Once pass and "Connected"
4. Close the WhatsApp in Multitasking Bar! Done Set A

Set B - iTools sync iPhone
1. Next, connect your iPhone to Mac launch iTools
2. Click on Applications
3. Find Whatsapp
4. Click on "Backup"
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5. Click "YES"
6. Pick Desktop, Click "OPEN"
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7. The iTools will backup it in IPA format on the  - Done for Set B
8. You can unplug the iPhone - Done set B

Set C - iTools sync iDevice for my case iPad Mini
1. Plug in the iPad Mini for my case
2. Click on Application
3. Click on Install
4. Find the WhatsApp Backup IPA you save  on the desktop. Select it.
5. Click "OPEN"
6. Wait the iTools process.  - Done Set C

Set D - The Setting - iPad Mini or iPad
1. Launch the WhatsApp on iPad mini
2. You will get a message, "Mobile Network Changed……"
3. Click "NO"
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TEST the "Chat" with your friends.
Limitation:
ONLY can connect to 1 iDevice.
Once using iPad, u can't use on iPhone until u verified the phone number again and this time they will sms you to verify using SMS.
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Itunes
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If you sync iPad to itunes, this message will pop-up. Just click "cancel".
I try to authorise, but cant. Because WhatsApp is for iPhone device.

FAQ- WhatsApp
So dont try switch here and there.... Unless you got 2 phone numbers....  
Credit to:
iTools for a great product!
Founders the trick! (Moon E Nell MAiC FB)
and of course WhatsApp!

*If this consider Jack Sparrow post, I will delete the post. Just sharing