In a break from all the APC angst..   This is the test environment I have set up, for the (2) APCs and the Raspberry Pi.   The Dreamplug is already connected separately via a separate JTAG/UART connector, and is headless, so doesn’t need the HDMI and monitor connections.

Typically, a 3-way USB/HDMI KVM would be used, but these are rather expensive, and the following setup works fine, and gives console access, which is essential for tests..

Linux / ARM Test Environment, connected to an X86_64 Linux main system..

The diagram should be self-explanatory.    The USB / UART converters attach to each board by tx  rx and gnd pins.   The Linux x86_64 host simply runs multiple instances of minicom, attaching to /dev/ttyUSB1 /dev/ttyUSB2  etc…   The Ethernet and USB hubs take care of multiple connections, and the basic HDMI switch – an inexpensive item – is the only device sometimes needing manual intervention (a 3-way toggle push-button..)    The device file designations are for a Fedora system, and may vary for other distros..