An example of how combo keys could be implemented via coroutines (untested).

ComboKeys.cs

using UnityEngine;
using System.Collections;


public class KeyCombo
{
	public float maxPause = 1.0f;
	public string[] keys;
	public string message;
	
	
	private int progress = 0;
	
	
	public void Check (MonoBehaviour owner)
	{
		if (progress == 0 && Input.GetKey (keys[0]))
		// Start tracking keys if the first key is pressed
		{
			owner.StartCoroutine (Run (owner));
		}
	}
	
	
	IEnumerator Run (MonoBehaviour owner)
	{
		float start;
		while (Application.isPlaying)
		{
			if (progress >= keys.Length)
			// If we typed all the keys, send the message and return
			{
				owner.SendMessage (message);
				progress = 0;
				return;
			}
			else if (Input.GetKey (keys[progress]))
			// If the next key was pressed, move forward and reset the timer
			{
				progress++;
				start = Time.time;
			}
			else if (Time.time - start > maxPause ||
			// Reset and return if we time out
				(
					Input.anyKey &&
					!(
						Input.GetMouseButton (0) ||
						Input.GetMouseButton (1) ||
						Input.GetMouseButton (2)
					)
				)
				// Or if an incorrect key was typed
				// NOTE: Might want to expand what is allowed here
			)
			{
				progress = 0;
				return;
			}
			
			yield return null;
		}
	}
}


public class ComboKeys : MonoBehaviour
{
	public KeyCombo[] combos;
	
	
	void Update ()
	{
		foreach (KeyCombo combo in combos)
		{
			combo.Check ();
		}
	}
}