1
general::
Together, jointly, as in They worked in concert on the script, or When mind is in concert with body, one can accomplish a great deal. This expression uses concert in the sense of "an agreement of two or more persons." [Early 1700s]
American Heritage Idioms
2
general::
Phrase(s): in concert (with someone)
Fig. 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.
McGrawhill's American Idioms And Phrasal Verbs