The PicoSplit consists of two halves, each equipped with a Raspberry Pi Pico. Both halves communicate with each other via a serial interface using a TRRS cable. Only one of the two halves has to be connected to the computer with a USB cable (the master keyboard).
This is the circuit diagram for the PicoSplit, with a total of 40 switches.
The wiring is the same for both halves except for the send and receive lines of the serial port. The half that connects to the computer gets a different firmware, but you can decide which half that should be. The master keyboard is the one which is connected to the computer. The other half is the slave keyboard.