I just want to know if it is possible to have on two phones the same whatsapp messages.
The short answer: you can't do it using the native app, but there is a web interface that will work on Chrome, Firefox, Opera and most other browsers in desktop mode (i.e. "request desktop site" or "user agent: desktop").
Whatsapp allows only one device to pull your messages from their servers. You can make the app think you only have one device by changing the MAC address and some other files using root access, but that doesn't mean it will work.
Whatsapp messages are stored only temporarily on the whatsapp servers. This means thay when device A downloads those messages, device B can no longer see them. This means that, in order to keep your messages synchronized, you will need to constantly sync your messages using tools with root access. This is not very usable to most people.
The easiest solution would be to use the app on device A and the web interface, (web.whatsapp.com), on device B. To make this work on an Android device you need to make it request the desktop site. There you can scan a QR code into your first device by opening the menu and selecting "whatsapp web". Point your camera at the QR code and you can use Chrome to send and receive whatsapp messages. This requires the device with the official app to be on and connected to the Internet.
Once your phone number is verified by WhatApp, whatapp will just keep sending the messages to your internet connected device from where you sent the message for verification. If you take a titanium backup of whatsapp and restore it to any device, it will start accepting messages from whatsapp.i have recieved messages on either devices and it worked if the devices are synced off line. I used one at a time. But never both of them together and think doing so will require re-verification of phone number. For receiving and sending messages and identifying your device whatsapp doesn't check phone number of device but the verification saved in your device.
is it possible to have on two phones the same whatsapp messages?
Whatsapp messages are stored locally on the phone, with periodic encrypted backups to sdcard. Its unclear that the messages are stored on the server once they are delivered. And even if they are, whatsapp doesn't provide a sync feature to sync messages to multiple devices as of now.
On a 1st device, when a message arrives, it gets stored locally on device and backed up to sdcard as a file. Say on the 2nd device (whatsapp cannot be used simultaneously actually), when a message arrives, same thing happens and u have a backup of file. So, you have got two files from each device, but you cannot restore messages from two backup files. And restoring messages is an option only when adding an account, not when using the app normally.
So, as of now, you cannot have duplicate message copies in both phones.
Can I use one number on two different phone(Device) on Whatsapp?
No, not atleast simultaneously. Ofcourse excluding
Whatsapp Web (see below).
Say you have logged in to whatsapp on 1st device. Then, say, you login to 2nd device and complete the verification process. Now when you open whatsapp on 1st device, it complains about whatsapp logged in from elsewhere and asks you to reverify. And if you reverify, the 2nd device gets unlinked, and it continues.
This can be used only if the browser and whatsapp linked device is side-by-side and the device has active data connection enabled. You need to scan the qr code displayed in the web browser from whatsapp on your phone (actually only this verification part requires devices side-by-side). Then you can use web version till the device looses data connection. The web version is a blue print from the device. You will see all the contacts and messages on the device in the web version. But considering qr scanning and always on data on mobile, the web version is far from multi-device usage.