This is possible even without account and sharing settings such as Authy's, for example using Google's Authenticator.
To add the account to two devices, all you have to do is add them at the same time. So you have to follow the usual method of adding an authenticator and when the time comes to read the QR code on the app you read it using both devices at the same time. When you do that you'll have the authenticator working on both devices and they don't have to be linked to each other in any way.