Boyce Avenue acted as vocal coaches to Fifth Harmony on 'X Factor,' and they're working together again, this time on a cover of Justin Timberlake's 'Mirrors.'

Fifth Harmony provided the -- you guessed it -- harmonies on the track, which is an acoustic rendition of JT's smash. Boyce Avenue's twang and guitars provide a country edge, but 5H provide pure pop perfection with their vocals.

"These girls are like sisters to us!" Boyce Avenue gushed on their YouTube page. "We can't believe we've known them and worked with them since their first performance ever together as a group."

It's clear that Boyce Avenue succeeded as vocal coaches -- even though these girls are dripping with natural talent -- just based on how far they've come ... and how beautiful this cover is. (Extra props to Normani Kordei for that whistle register ad lib. Wow!)