Just tested this code. Works correctly. Have second heatbed set as C (so octoprint will show it). If heater_bed get there first waits for bed2. If bed2 gets there first waits for heater_bed.

command
sync_heater_temperature heater=bed2 to=heater_bed

heater_bed and heater_generic in config file

[heater_bed]
heater_pin: P2.5
sensor_type: EPCOS 100K B57560G104F
sensor_pin: P0.25
control: pid
pid_Kp: 171.72
pid_Ki: 31.76
pid_Kd: 619.01
min_temp: 0
max_temp: 130

[heater_generic bed2]
gcode_id:C
#The id to use when reporting the temperature in the M105 command.
#This parameter must be provided.
heater_pin: P2.4
#max_power:
sensor_type: EPCOS 100K B57560G104F
sensor_pin: P0.23
#smooth_time:
control: pid
pid_Kp: 171.72
pid_Ki: 31.76
pid_Kd: 619.01
#pwm_cycle_time:
min_temp: 0
max_temp: 130
#See the "extruder" section for the definition of the above
#parameters.

Thank you for your feedback. Maybe it would be interesting to add attributes for heater_generic to synchronize when Klipper starts?

[heater_generic]
...
sync_heater: heater_bed
#sync_temp_offset: 0.

Is the temperature offset useful for your usage?
Can this/these parameters be useful for other heaters than [heater_generic]?

I prefer to wait for Kevin's comment before going further in this developpment.

Thanks. FWIW, I'm not sure of the advantages of adding this code as opposed to using macros that obtain similar results. I'd expect that there is a relatively small audience of users that would want to synchronize heaters, and I expect those users have fairly custom setups that wont be easy to model with simple commands/config options. Those users may find macros easier as they explicitly state the actions to take on each request.

-Kevin

Thank you Kevin.
It's not a big deal to do the heater synchronisation in macro. @devond76180, for a heatbed, override M140 and M190 with macros. Use the "rename_existing" argument and as gcode, call the renamed M140/M190 and SET_HEATER_TEMPERATURE for the second heater. I should push macros samples soon in #4464 for the extruders that can help you.
The synchronization of the extruder steppers will still be missing. There is currently no solution for this.

In macro you can use the G-code command TEMPERATURE_WAIT. I agree with Kevin that the sync_heater part of this PR adds no feature. This only adds convenience.

Replaced by PR #4489 for syncing of extruder steppers and macros for syncing of heaters.