Based on a statement by wabetainfo, it seems that WhatsApp could allow users to have their account signed in to up to 4 devices at the same time. While there is no news on when this feature will be available, this means that signing into another device will not sign the user out of the previous device they were using.
However, it is best to note that even if the multi-device feature were to be available to the public, chances are not all features might not be utilized right away. Not only that, do take some of the information with a grain of salt, due to the fact that Facebook has yet to provide an official statement on the feature mentioned above.
