I want to build a device with both a small screen and 4 digit display. I have been having trouble getting the 4 digit display to work with my model B+, but it works with the my model A, you can find more detail about my issues in this question. I'm still working on that, I'm just slow.
Anyway it occurred to me that my case can hold two pis, I could use the model A to run the display and the model B+ to run the screen. To get more out of this I could attach a camera board to each to build a 3D surveillance system. Now I could run the two systems indepedently by putting a wifi dongle in each, but I also thought I could wire the two machines together through GPIO and pass data back and forth. At a minimum I'd need to get the time from the model B+, which would get the time from the internet, then pass it to the model A to run the 4 digit display. At most, I'd like to pass the still images or data from the model A to the model B+ for access through the internet.
What protocols (if any) exist for Pi to Pi communication through GPIO? What kind of transfer speeds should I expect? Am I better off with 2 independent machines in 1 box?
On a side note I built a switch that lets me hook 2 Pis to a single screen, then toggle which one gets displayed. Might be helpful in this situation where I can see what each Pi is up to instead of only being able to see the model B+.