Nope, I'm not asking why I don't experience your mind. I'm asking why there is any differentiated experience at all, since the experience is going on in the Being.
It's not "going on in the Being". Why does experience have to "going on somewhere"? Experiences aren't
anywhere. They are not "in your head". They are not "in Being". They do not have a location. They certainly do not have a physical location and Being doesn't have a location at all. So what is "in" supposed to mean in your question?
You can think of it in terms of a shared homunculus if you like.
homunculus/being ---> Paul's "noumenal brain" ----> What it's like to be Paul.
homunculus/being ---> Geoff's "noumenal brain ----> What it's like to be Geoff.
You are thinking that because it is the same homunculus, that it should follow that there should be only one being and that it should experience being everything at the same time. But the system does not specify "what it is like to be the homunculus without also being either Paul or Geoff." What you are trying to ask me is "What is it like to be the homunculus?", especially since one of the manifestations of that homunculus is YOU. But I never even said there was anything like "what it is like to be the homunculus". The homunculus on it's own, without any noumenal brain, can't be aware of anything at all. It would be a bit like a film projector without a film in it. You can switch it on, but nothing will appear on the screen. So there isn't any reason to believe there is a "what it is like to be a homunculus." You are asking me a variation on what you asked me before "What is it like to Be Being?" The question doesn't make much sense. Although Heidegger tries to answer it anyway in the form "What is the Being of being".
Put more simply, without a noumenal brain you cannot have a mind at all because there is no content to be had. But as soon as you DO have a noumenal brain, and there is a mind,
it must be individuated because it is partially dependent on the brain.
In Heideggerian terms - you are dasein, not Being. You are a being-in-a-world.
To be differentiated, the experiences must be going on in the noumenal brains, at least to some degree. Are they? If so, why do we need Being at all?
Experiences aren't "in" anything.
How can there be any spatio-temporal experience at all, if it is not grounded in the noumenal?
It
is grounded in the noumenal. Just like physical things are grounded in the noumenal even though the noumenal isn't physical.
Does the noumental/Being experience engine just make it up out of whole cloth?
Don't understand the question.
If so, then clearly my experience of the physical does not map in any straightforward way to the noumenal.
It maps straight on to something in the noumenal brain. I can't tell you what that something is.