Helper singleton for running multiple coroutines simultaneously, and then waiting for all of them to finish.

CoroutineSyncer.cs

using UnityEngine;
using System.Collections;
using System.Collections.Generic;

public class CoroutineSyncer : MonoBehaviour
{
	static CoroutineSyncer m_instance;
	public static CoroutineSyncer Instance
	{
		get
		{
			return m_instance;
		}
	}

	int m_latestId = 0;
	Dictionary<int, int> m_leftForID = new Dictionary<int, int>();

	void Awake() {
		m_instance = this;
	}

	public IEnumerator DoSimultaneously(List<IEnumerator> coroutines)
	{
		int localID = m_latestId++;
		m_leftForID[localID] = coroutines.Count;
		for (int i = 0; i < coroutines.Count; i++)
		{
			StartCoroutine(Wrapper(coroutines[i], localID));
		}

		while (m_leftForID[localID] > 0)
		{
			yield return null;
		}

		Debug.Log(localID+": Done all!");
	}

	IEnumerator Wrapper(IEnumerator coroutine, int id)
	{
		yield return StartCoroutine(coroutine);
		OnComplete(id);
	}

	void OnComplete(int id)
	{
		Debug.Log(id + ": one is done");
		m_leftForID[id] -= 1;
	}
}

CoroutineSyncerTest.cs

using UnityEngine;
using System.Collections;
using System.Collections.Generic;

public class CoroutineSyncerTest : MonoBehaviour {

	void Start() {
		var operations = new List<IEnumerator>();
		for (int i = 0; i < 10; i++)
		{
			operations.Add(AsyncOperation(Random.Range(2f, 10f)));
		}

		StartCoroutine(CoroutineSyncer.Instance.DoSimultaneously(operations));
	}


	IEnumerator AsyncOperation(float time)
	{
		Debug.Log("Waiting for " + time);
		yield return new WaitForSeconds(time);
		Debug.Log("Done: " + time);
	}

}