It's not exactly "unison" (it is indeed very tightly coordinated though). The engines will be lit in pairs, with a couple dozen milliseconds between each set starting up.
Honestly; thank you for taking the time to spread the knowledge.
Here's a source on the engines being started in pairs: https://www.nasaspaceflight.com/2017/11/spacex-aims-december...
"Instead, Falcon Heavy will take a page from the Space Shuttle’s book and will employ a staggered start sequence – like main engine start on Shuttle where each main engine started 120 milliseconds after the previous.
For Falcon Heavy, it is understood that two engines will be lit simultaneously followed by the next two… and so on until all 27 are up and running."