Run Parallel Apps
Did you know you can install, run, and clone apps on your OnePlus 5T phone? Yea, you can have two Facebook apps with two different accounts signed in.
With some phones, you can clone any app and have two, but OnePlus limits this to social media apps. This is to prevent unauthorized usage of certain apps, and because it makes more sense to have two of the same social media app.
If you have two social media accounts go to Settings > Apps > Parallel Apps and get started. From here you can clone an app, sign in with two accounts, and switch between them on the fly just like switching between any other applications. It's pretty powerful and useful. Open the app tray, and you'll see two of that application.