Viber for iOS is optimized to run on iPhone devices and only on these devices. Of course, since iPads share the same OS as the iPhones, you can install Viber on these devices as well, but with limited support in which case you might experience issues (disconnection, login, chat delay, etc.). The first time you create an account on Viber, you will also receive a Viber ID which is generated based on the phone number and the device (model, OS, IMEI, etc.). Their policy prevents using the same Viber ID on multiple devices (eg. same account on iOS, Android, Windows Phone, etc.). Because of this limitation, you can't have the same Viber ID on multi-platforms. However, things change when it comes to mobile platforms and desktop/laptop platforms like Windows or Mac.
On a recent update, the company said that you have the possibility to activate the same Viber ID only if the latest version of Viber is installed on the device where the Viber ID was created, then when you install and activate it on tablet, the app will ask you if you've previously installed Vibe and where. It's a new change that works for Android (phone and tablets) and Windows (phone and tablets).