When two or more people know each other so well they either finish each other's sentences, have tons of inside jokes they only understand, can communicate without even using words, or talk in a way they only understand either through some made up language or talking extremely fast or all of the above.
Sync Buddy 1: Man i really want...
Sync Buddy 2: To get some funnel cake at the fair?
Sync Buddy 1: YES!
Sync Buddy 1: Hey bro I'm going to BASKIN ROBIN
Sync Buddy 2: LOL!
Dude 1: Wtf?
Dude 2: Don't worry their just sync buddies