I have not used a HC12 (only nRF24L01+ modules) so I don't know if it has an automatic acknowledgement system that would allow the sender to know is a message was received.
you would have a reasonable guess of the clock offset of the timer in transmitter2.
I reckon if the sender includes its own millis() value at the time of sending there is no need to figure out the clock offset.
If you know the other transmitter sends data only on the half second, you already know the other transmitter's timestamp.
Only if a message is never dropped.Or what happens if the sender is reset without going through a new SYNC routine?...R
Serial.print('<'); // start markerSerial.print(value1);Serial.print(','); // comma separatorSerial.print(value2);Serial.println('>'); // end marker
Still the delta value increases with about 2 ms every second.