There are over a million hits in a Google search for 'as if in concert', probably making it idiomatic, though I'd say a fixed expression rather than an idiom. This is arguable, as the expression is obviously used metaphorically, for non-musical events.
Only the 'in concert' string is easy to find in dictionaries, but then I'm guessing that most answers here will need the 'as if' qualifier.
in concert ... [figurative]
in cooperation with someone; with the aid of someone.
- Mrs. Smith planned the party in concert with her sister.
- In concert they planned a lovely event.
[McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.]
As if in concert, John, Ali, Jill and Sue turned up at the fancy dress party all dressed as Cousin Itt.