WhatsApp has been prepping support for multiple devices for months now — even longer if you consider its semi-functional existing web and desktop implementation as an intrinsic part of the process. The feature started to go live for some users a few days ago, and appears to be spreading widely now. Most beta app users should have it now, and it is making its way to non-beta users too. It's trivial to set up and, although most functionality is retained, it's still a little disappointing.
WhatsApp has managed to reach a worldwide audience and break through 2 billion monthly users despite a severe handicap: The app doesn't support multiple devices for the same account. One phone number equals one phone or one tablet, that's always been the equation. And while there are workarounds to allow two different numbers on the same phone (for dual-SIM devices or Business users), there hasn't been any trick to make the same number available on two phones or tablets simultaneously. The great news is that WhatsApp is already working on this.