To say something in choir word

There is another, rather more esoteric, answer: "in concert". It's more poetic, less likely in everyday speech, and it means "intentionally acting together so as to have some effect".

By contrast, "in unison" need not have intention: two people may accidentally speak in unison, but they can't accidentally speak in concert or in chorus.

"In chorus" need not carry the idea of aiming to carry out a goal. Several people may speak "in chorus" (for example, the unison greeting of a class to their teacher), but to speak "in concert" there must additionally be some reason why speaking together is necessary to achieve something.

Two people may speak "in concert" if they have arranged to speak at the same time for added effect, for example. It doesn't often refer to speech, but more usually to an action of some kind.

I think this is actually in chorus. See the second example here:

chorus

3. A simultaneous utterance of something by many people.

   a growing chorus of complaint
   “Good morning,” we replied in chorus

Oxford

I believe Andrew's answer was spot-on.

Here is one more suggestion:

In unison.

At the same time; simultaneously.

Origin: from Latin uni- "one" + sonus "sound".