@ericski
I have the gerbers and DipTrace PCB files for the proto board I show in the earlier post, or I can take a better photo of that one which shows the layout in more detail.
The USB is switched by a DP4T switch (seen in the picture) this swaps the TX/RX from the USB to the RX/TX of each processor. The power, ground, and reset from the USB are wired to each processor. This means when you're uploading a sketch, all the CPUs reset and the one selected by the switch starts receiving the sketch, while the others start running whatever program is loaded.
Hope that answers your question.