The oldest and best tried 2-wire comms with power is a telephone line. You
can signal using tones or interruptions (like pulse dial).

The easiest communication is using RS232 (inverted or not) at baseband (no
carrier).

If you need to update clocks with setting info you can send a short burst
at say 19200 once a second or once a minute.

The bidirectional part is handled by all the parties listening all the
time (including to its own tx). Thus a jammed transmission will be
detected and the sender can reschedule and retransmit it easily.

Peter

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