All 8 Uses

unison
in
Red Queen
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"Not a wise time to start picking Silver pockets," the twins say in unison.†
p. 40.2

in unison = together at the same time
All over the spiral, the leaders of the different houses shout in unison.†
p. 64.3
"I can do it," Cal and Maven say in unison.†
p. 109.7 *
"Lady Mareena," they say in unison, bowing stiffly.†
p. 122.1
"Very well, sir," the Sentinels say in unison, unable to refuse a prince.†
p. 142.1
"We will rise," he breathes, in unison with Tristan.†
p. 164.5
They take aim mechanically, raising their guns in unison.†
p. 376.6
In complete unison, the sobs and sniffles through the chamber die out.†
p. 256.2

Definition:

being together and in sync (doing or saying the same thing at the same time)