It's a standard feature on Xiaomi phones. You can set up two user profiles, each with a different lock screen password. Each profile is effectively a different phone - different apps, different settings, even a different SIM card if you like.
Really? Guest mode is a standard feature since Lollipop, but that just logs you into a completely blank account without any installed apps or user data. You can switch Google accounts, but that's a multi-tap process that can't be activated directly from the lock screen.
Second Space isn't unique in functionality, but it's exceptionally slick in execution.
http://xiaomitips.com/guide/how-to-use-miui-8-second-space-f...