Interesting questions. My guess is that the “shield” is unbreakable but the victim can be tricked into that it’s not and that way someone unfriendly can enter. If they are aware of it they would probably notice like if they are dreaming or having an AP but not if they’re fully asleep.
I’m basing these guesses on what I’ve read from others, haven’t experienced it myself.
I believe that it’s impossible to break the “shield”, people would then be able to bug other people in their sleep, how about bugging the president? >.< There would be severe concequences…
And if it was possible wouldn’t our ancestors who dealt with dreams everyday found out?
Being able to meet someone in the dream without the other persons approval would mean that we could influence them into deeds they even wouldn’t be aware of doing, precisely like in the movie Inception 
This makes perfect sense to me! Especially with my new theory, what you have said adds on and fits perfectly into my theory of shared dreaming. Yes shared dreaming is possible and there most definitely is a shield that prevents shared dreaming, I believe I and everyone else can pass this shield and have a shared dream and I want to try this as soon as possible.
Whoever can LD on command, was a natural, or is very experienced in LDs, do me a favor please and tell me your DG or SG in description please, I will try this myself, I will try to pass the “shield” that bezjezt theorizes of, and try to find you and make you lucid if you are not already lucid. I want to try to have a shared dream. Please know though, this would work better if I pick someones DG who has insanely good DR that if I had done this in my LD and if it became a shared dream, he would not forget it. But even then, tell me all your DGs anyways and I will pick one to attempt shared dreaming.
Hmmm, interesting posts. For one, I agree with Dreamwalker, if you CAN break this “shield”, there might already be some sort of havoc in dreamworlds, and I would think that our more advanced ancestors (wow, lol that sounds like an oxymoron! More advanced in … spiritual matters) would know a lot about this.
As for darksage’s post… Like I said, if it is possible, I agree, and it would seem to explain some things about SD’s. But I suppose, to prove anything, that’s exactly what you would need. Two people who are experienced LDers to attempt this. Unfortunately I myself can’t do this, as I’m not at all consistent, but I would like to when I get the chance. Otherwise, if any others want to try this, and see if there is some shield and a way to get “around” it… you could post any results here…
Umm, not to contradict you but it’s proved we only use about less than 10% of our brain, and the other 90% is completely unused, but dreams COULD use it? Aka SD’s
From reading this post, I feel you will be able to have shared dreams in the future, this is all in my theories I come up with on the spot though. You will become good at LDs in time, its only up to you of how much you practice etc… and how good you become at LDs. Once you become that good, you dont have any mental blocks about shared dreaming and with practice, you can become good at shared dreaming just like you become good at LDs. The reason you can come up with cool concepts and theories that could work in many different ways to have shared dreams without ever having them, is because you just unlocked knowledge you didnt know was there, the knowledge we all already have, and you did this all on a subconscious level by not limiting yourself to believe shared dreaming is not possible to begin with. So within my theories, I am saying if you ever in your life become good at LDs so that you have a solid LD rate, you will be able to practice shared dreaming and eventually have shared dreams at the same rate you have LDs buy applying the same amount of practice.
Unforunately, I hear those statistics are wrong. (I’m not completely sure if its not or it is, its hard to know anything for sure on the internet 
) But I think there was still something about not using the brain to its full potential. If that is true, that may help in the case of SD’s too…
Yeah… that quote is completely wrong - ever see an MRI scan of a working brain? Definitely more than 10% of your brain is being engaged just with very simple tasks. I could go into more details as to where the quote came from, how old it is etc… but its a bit off topic. But just to point out, its not a really good base for any arguement.
I think if you want to try your shared dreaming ideas out - maybe you should start with having shared thoughts first - something to do in the conscious world??! Just an idea - that way you are consciously opening up this ‘barrier’ you are reffering to… soon enough, you will work out if your SD ing is for real or not.