Two players on one computer

Open the 2nd lichess account from an "incognito mode" browser window.

Ctrl+CapsLock+N should open the incognito mode, where you could login in a different lichess account still using Chrome.

Once established the game, you will have to switch between the two browser windows, turn by turn.

thank you very much Lucianavm, your idea is nice, and it works.
From now on, I'll be able to teach chess to a beginner friend on lichess, when I'll be with him