I assume that the light and the 2 sensors are of traditional type (i.e. not accessible via FHEM).
To notify FHEM (or another dimmer) about "light goes on" I see three possibilities: measure the signal between the sensors and the light, measure the energy change when the light goes on, or measure the light intensity change. Sending the signal to FHEM can be done via different methods like WLAN or ZWave.
Which of the possibilities is the easiest/cheapest, depends on the setting (how are the sensors and the light installed), how you value your time spent on tinkering vs money investment, and if there are design constraints like the changes should not be visible or they must be aeshtetically pleasing.
I would probably go with your suggestion.