That’s good enough.
It seems the best option is probably a template trigger on attribute hvac_mode.
{{ state_attr('climate.TRV1', 'hvac_mode') == 'heating' or state_attr('climate.TRV2', 'hvac_mode') == 'heating' }}
This should trigger when one or more is in heating mode.
Or you make this a template binary sensor and that way it should be on or off and you can trigger the automation on any state change of the sensor and just use the {{ trigger.to_state }} to turn on or off boiler.
The
? is probably not a valid character so don’t use it. You could probably name it TRV group or something. That would make sense.