div[data-controller="hoge fuga"]
  button data-action="fuga#fugaAction"

app/javascript/controllers/hoge.js

import { Controller } from "@hotwired/stimulus"

// Connects to data-controller="hoge"
export default class extends Controller {
  initialize() {
    this.element[this.identifier] = this 👈これが重要
  }

  hogeAction() {}
}

app/javascript/controllers/fuga.js

import { Controller } from "@hotwired/stimulus"

// Connects to data-controller="fuga"
export default class extends Controller {
  fugaAction() {
    this.element['hoge'].hogeAction() 👈これでhogeのアクションを呼べる
  }
}