One morning, I woke up at the same time as my girlfriend. I mentioned dreaming we were in an old hotel. She was quick to ask me to tell her more about it, because she had just awoken from the same dream!
We compared notes: I asked her why I never saw her there until the second half of the dream, and she answered that she spent the first half of her dream driving around Dallas looking for the place, stating, "I had no idea you were going to stick it way out in the country." I asked her why, when she got out of her car and I waved to her from the roof where I and others were working, she didn't wave back. She answered that she saw the men working on the roof but didn't know I was one of them nor did she see anyone wave to her. I asked who that slim girl with very long black hair was who got out of the passenger side of her car. She looked at me with true astonishment, then exclaimed, "That was my best friend from Oklahoma whom you've never met!" In my dream, the girl had later come up to me alone to shake my hand, introducing herself as my girlfriend's close friend.
My girlfriend asked me questions as well about things I had not yet mentioned, such as that a number of rooms had no Sheetrock on the walls between them, only the 2X4 wood framing, allowing us to walk through those walls between rooms. This was one of several proofs she really did share the same dream.
More than a year later, she and I actually found that hotel, bought it, and moved in! And yes, there was no Sheetrock on the walls between rooms on the second and third floors, allowing us to walk through the walls from room to room. They had never been finished. Also, I later actually sat on the exact spot of the roof from where I'd waved to her when she got out of her car in the dream. I was up there because it was the best spot for drying items in the sun. But in real life, I was totally alone; no workmen were beside me nor was anyone below near the street.
Sharing dreams with someone isn't a rare occurrence. There is a book on the subject titled Mutual Dreaming: When Two or More People Share the Same Dream that contains over two hundred accounts from others who have had a similar experience.