You will still have to write code that handles contentions to make sure that valid data is being written and read. I am not sure what this involves

That is actually dead simple. Reserve on BYTE (never a bit on a DPM) that reads 0xff if A is done writing, 0 if not and another byte for the other direction. Then, if you never write except on request from the other side you are home free. As for the prices Lee found, he is assuming that all RAM should be dpm, there is no need, you can page the RAM and use a very small DPM for communication.

Erik