Что, если я добавлю несколько зависимостей во второй параметр React useEffect?

Он будет срабатывать при изменении любого из них. Можно подумать, что вы говорите React:

a and b are the things that I am using inside this effect, so if either of them change, my effect will need to cleanup the old version and re-execute with the updated values.

Вот простой пример, который позволяет увидеть поведение:

import React, { useState, useEffect } from "react";
import ReactDOM from "react-dom";

function App() {
  const [dependency1, setDependency1] = useState(1);
  const [dependency2, setDependency2] = useState(1);
  useEffect(
    () => {
      console.info("only dependency1", dependency1, dependency2);
    },
    [dependency1]
  );
  useEffect(
    () => {
      console.info("dependency1 and dependency2", dependency1, dependency2);
    },
    [dependency1, dependency2]
  );
  return (
    <div className = "App">
      <button
        onClick = {() => {
          setDependency1(prev => prev + 1);
        }}
      >
        Change dependency1
      </button>
      <button
        onClick = {() => {
          setDependency2(prev => prev + 1);
        }}
      >
        Change dependency2
      </button>
      <button
        onClick = {() => {
          setDependency1(prev => prev + 1);
          setDependency2(prev => prev + 1);
        }}
      >
        Change both
      </button>
    </div>
  );
}

const rootElement = document.getElementById("root");
ReactDOM.render(<App />, rootElement);