WhatsApp on multiple devices (and without the smartphone): how to do it on Android and iPhone

After a long wait, WhatsApp has finally activated the Beta for the multi-device functionality on the iOS and Android apps of all users: by subscribing to the test program it will be possible to use the Web or Desktop versions of WhatsApp on up to 4 different devices. The difference from the current version is that you no longer need to keep your main phone connected to the Internet at all times.