1 Viber account on 2 devices?

In December last year I managed to get my viber account on my iphone working simultaneously on my samsung tab 3 so when someone sends a message it appears on both devices. I can't remember how I did and have been trying to do the same for my girlfriends Ipad and android phone. When I activate one device it deactivates the other which makes sense but can't figure out how I got both mine to work. Has anyone else got one account working on 2 devices simultaneously? If so can you advise me on how you did it please? I think It was something to do with the activation codes if I remember correctly