As you can see in this post, I’ve been trying to figure out how to do this for some time. The failures in production show us that you cannot simply hook up two chips together because the I/O pins connected together can fry the other chip. We found that we would need to completely isolate the two chips using 16 channel multiplexer. Which given the size of the product would have been nearly impossible to place on the board not to mention a 4 layer PCB. Designing this board would have taken another month and pushed us past our deadline which was January.
The goal of Arduboy is one of learning and community. We are here to help each other have a good time with video games. If that’s not whats happening let me know and we can get you sorted out.