The two Pixhawks share a GPS, telemetry and the SBUS RC receiver.
The GPS TX line is connected to both Pixhawks, RX only to one. One Pixhawk has a telemetry radio and they are connected together via “telem2”. I only use one power module at the moment, so only the boat Pixhawk knows when the battery runs dry. I guess, I could split the signal lines from the power module to both Pixhawks, too.