I am not a lawyer and I'll yield on this to anyone with relevant legal knowledge. Nor have I seen the movie so the quote as given above is all I have to go on. That said ...
I am not aware of "murder in concert" having any technical legal meaning. More likely, the movie is simply using "in concert" in the ordinary English sense. Not referring to a musical performance, but another definition of "concert" is "to act together". If you say that two people did something "in concert" you mean they co-operated with each other. They worked together or otherwise co-ordinated their actions. Or you can say that someone performed some action "in concert with" some other event or some idea or larger plan.
I'd have to see the full quote to know exactly what is meant. But it would be ordinary English to say, "Bob planned this murder in concert with George", meaning that the two of them planned the murder together. Or, "Bob planned this murder in concert with his scheme to scheme to embezzle the money", meaning it was part of this larger plan.
Tangential but potentially amusing side note inspired by @konradViltersten
When I say "ordinary English", I mean as opposed to technical legal language, not necessarily what ordinary English speakers would say in every day conversation.
It reminds me of a story I heard from a mathematician years ago. He went to a conference and attended a lecture where someone was presenting a mathematical proof. At one step in the proof he said, "And of course it's obvious that ..." and transformed the equation for the next step. Someone in the audience objected, "Wait a minute, I don't think that's obvious at all." The speaker started to reply, stopped, stepped back, looked at his proof, and then turned and walked out of the room. Everyone sat rather confused for a while but he didn't come back.
My friend happened to be in the same lecture room on the last day of the conference when this earlier speaker suddenly walked into the room. He looked like he hadn't slept or shaved for several days. This was in the days before PowerPoint so the room had blackboards filling 3 walls. Totally ignoring the current speaker, he walked up to the first blackboard and started writing, He filled all 3 blackboards with formulas and equations and when he got to the end, he threw down the chalk in triumph and declared, "There! It's obvious!"