This word, or expression has been bugging me. I think it's "to say something in choir", though I'm not sure, and everything I get from searching the internet is videos of choirs singing. The expression I'm looking for is someone saying something at the exact time. Doesn't need to be intentional, just that someone responded at the exact same time. The sentence "to say something in choir" is directly translated from the expression in Norwegian, as that is my original language. "To say something in symphony" might be it. Though, this has nothing to do with music. Simply two (or more) people who are saying something simultaneously.
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:
A simultaneous utterance of something by many people.
a growing chorus of complaint
“Good morning,” we replied in chorus
I believe Andrew's answer was spot-on.
Here is one more suggestion:
At the same time; simultaneously.
Origin: from Latin uni- "one" + sonus "sound".
Most likely not the answer you're looking for, but one that most children will be aware of is:
A jinx occurs when two people say the same words, in unison accidentally. The first person to say 'jinx!' following the occurrence has jinxed the other person who is then not allowed to speak until someone says their name.
Of course, if both people who spoke the first in unison sentence know the game, they are both likely to say 'jinx' at the same time as well. Depending on the variant, the procedure then is to say 'double jinx' (followed by 'triple jinx', 'quadruple jinx', 'jinx infinity' or similar progression), or count to ten out loud and then say it again. This invariably leads to an extended sequence of two people saying what appear to be random words in concert.
EDIT: Oops, as @AndrewT mentions, the general use of the word 'jinx' is to cause bad luck or unfortunate circumstances on another: you may need to provide extra disambiguation when using it.