Duet is a TikTok feature that’s been a community favorite since the start and one we’ve recently revamped with exciting new layouts. Duets allow you to build on another user’s video on TikTok by recording your own video alongside the original as it plays. Users have the option to select from one of four new layouts: a left and right layout, react layout, top and bottom layout, and three screen layout. You can also turn on your microphone as you Duet, making it easy to add your voice to every new content remix.”]
Source: https://newsroom.tiktok.com/en-us/feature-highlight-new-layouts-for-duet