The latest beta version of WhatsApp for Android has a dual panel interface for tablets. What usually happens in these cases is that new features like this are tested in the Beta Channel for a while (weeks or months) and eventually affect everyone.
I hope the same logic applies here. A dual-panel UI on a tablet makes a lot more sense than just a scaled-up version of the phone UI.
The first version of WhatsApp compatible with Android tablets was released to beta testers last year, but without split view. In the new screenshot above you can see the new UI that we believe is currently being rolled out for beta users. Not surprisingly, the chat list is always on the left, allowing you to quickly switch between conversations without having to go back to the beginning. Split view is also available on the Calls and Status tabs.
Hopefully this gets out of beta testing soon.