http://wccftech.com/galaxy-s8-portrait-mode-update/

Galaxy S8 ( Amazon, $639.00) lacks a dual-camera, which is exactly the kind of smart business strategy Samsung laid out in order to force consumers to purchase the Galaxy Note 8 in order to get their hands on exclusive features such as Portrait Mode and optical zoom functionality.

However, the Portrait Mode uses software and camera hardware tricks in order to blur the background from the foreground, making the latter appear more prominent and from what we have seen from the Pixel 2 camera, you honestly do not need a secondary shooter to make this happen.