Interesting question! It wouldn't be too difficult to fake things, we'd setup a dedicated email alias for each candidate so "$id@jobs.example.com", but it wouldn't be impossible for people to get a friend to do the first part.
I know it crossed my mind that multiple people could be sharing a keyboard during the blind-sessions.
Do companies hiring exclusively via video calls (due to covid) actually make sure that the person is the same in each round? Or is the same person who shows up on day one?
The probability is being caught is much higher, and therefore reduces the probability of people trying to cheat. I also imagine you would be interviewed by people you are working with, so clearly they would notice in day one.