Watching a bit of Patrick Stewart's Macbeth yesterday, I was reminded of something I don't like about the Porter's scene. Whoever is at the door knocks *10* times. That's an awful lot of knocking. If you were at somebody's front door, you'd almost certainly give up before knocking 10 times. I realize that this is a castle, not a house, and that someone is surely home and just needs to wake up. That doesn't change the fact that the amount of knocking is jarring to me, it takes me away from the scene and makes me think "Somebody answer the damn door!"
Here's what came to me, though. The Macbeths hear 4 knocks before exiting, and then the porter hears 6. But what if the first knock that the Macbeths hear is really the same first knock that that the porter hears? They are almost certainly in two different parts of the castle, after all. See what I'm saying? What if these two scenes actually take place simultaneously? It's a common enough trick and you've probably seen it in any number of novels: one chapter shows you that a situation has changed unexpectedly, and then the next chapter, written from a different character's perspective, goes back in time a little bit and shows you how that character caused the change in whatever situation.
How might such a technique play out on stage? Could you even attempt to put both the Macbeths and the porter on stage at the same time, or would they step on each other's lines? If you don't, though, how do you explain that this is not a sequential series of events, but a simultaneous one?