At this stage, one Viber number can be assigned with only one device simultaneously. When you try registering to Viber with the same number on a second device, the first device immediately deactivates, and its data (message history, Recents, etc.) is deleted.
What you can do, however, is register your second device with a different number: your home number, an old cellphone number, family's or friends.
In future versions we are planning to add an option to register Viber on multiple devices. Once we have news on that, we'll let you know 🙂