That thing in videos where one person is dancing slightly in front of another person is like the visual equivalent to the "chorus" effect in music studio recordings
Basically, in audio recording if you have two similar but slightly different takes, it adds sparkle to the sound, it makes it seem bigger. Similarly with these sorts of tik tok dancers (to me) it has a similar impact, but visually instead of audibly.
I could be mistaken but I thought they got their name from actually singing too? I believe the name for the "chorus" effect in audio recording comes from that experience - hearing the sound of a group of people singing instead of one person singing. Regardless, the name is appropriate.