Since you're using Action Sequences, you could also use Eval commands in there to execute code. But we'll use a Pre-Damage Eval just for starting simplicity.
The instructions from Irinia on how to create a multiple battler cutin say:
Code:
var battlers = [];
battlers.push($gameActors.actor(1));
battlers.push($gameActors.actor(2));
battlers.push($gameActors.actor(3));
battlers.push($gameActors.actor(4));
var settings = $actionCutinSettings($dataSkills[10]);
var template = 'Horizontal';
$actionCutinTemplate(battlers, settings, template);
It then explains underneath what each of those lines means. Since the Unison Attack gives you ways to reference the attacking actors, which you're already using in both your damage formula and action sequence, you'd just plug those variables in instead.
Code:
<Pre-Damage Eval>
var battlers = [];
battlers.push($UnisonMember1);
battlers.push($UnisonMember2);
var settings = $actionCutinSettings($dataSkills[10]);
var template = 'Horizontal';
$actionCutinTemplate(battlers, settings, template);
</Pre-Damage Eval>
I have no idea what the ID of your skill is because you cropped your screenshot, but you would replace the skill ID 10 with whatever it is.