Trying to nail this one: yes, two or more people can have "shared dreams" where a part of our consciousness somehow detaches and meets the consciousness of the other people involved Elsewhere. Regardless of where our bodies are physically located. Not a new idea; a lot of strands of esoteric thought hold that this is perfectly feasible and it happens. Many people report experiences like this. (Again, the beauty of Forteanism: we can have ideas like this, speculate on them and explore where the speculations lead, but without feeling as if we have to make a belief out of it.) A general idea about the Astral Plane is that it stands outside time as we experience it and different rules apply. Robert Anton Wilson coined a word for it: the Heutemorgensgesternewelt, the "today-tomorrow-yesterday world" where linear causality either doesn't apply or it's a lot looser. So I'm inclined to suspect that if meeting other people on the Astral is possible and a dream becomes more than subjective, a shared experience, then we don't all go to that shared place through the same door. From my point of view I might be meeting you in there tonight; you might remember the dream from last month; a third party involved might not recall it until next year. What the triggers are for this I can barely speculate about, except wondering if emotions play a part. I might dream tonight abut a situation thirty years ago which had a lot of emotional intensity at the time. Another person involved in that situation might have the same shared dream, but from their point of view they've already had the dream twenty-five years ago... and we meet in a place where all that is irrelevant. (I once met myself in a dream.. looked into a mirror and saw an older me. That was freaky, even more so when one morning I looked at my reflection in a mirror, wondered where I'd seen that face looking back at me before... and remembered the dream, which I'd had maybe fifteen years previously....)
And that first alien abuction dream I described a post or two back involved, or may have involved, a girl I wasn't due to meet in this world for another five years....