How interesting that Joan chose to go live in another house considering how close the two cats have always been. It must be sad for you I am sure.
We had a cat do the same thing but he came to live at our house. He lived on another street behind us and steadily chose to stay on our porch all the time. One day the neighbor came to us and asked if we would keep the cat. They had been wanting to move to a retirement village and would not be able to take him with them. I think the cat knew way ahead of time and made the transition to our home easier on them. We had the cat for another 3 years before he died at 19 years old. I just thought I would share that with you.
I am sure he will come and visit you often. ;-)