Let’s have 2 points A and B and point C exactly in the middle. We can indirectly synchronize the clocks at A and B by sending the signals to C and calculating back the time on A and B clocks so the signals from A and B would arrive at C at the same time. Let’s assume temporarily that uni- directional speed of light is the same in both directions and equals c.

We can now synchronize the times at A and B with C knowing that at the moment signals were send towards C (assuming the signals arrived at C at the same time) the time at A and B should be same as time at C minus travel time of the photon from A (or B) to C.

Now we can send the signal from A and B towards C at the same time as indicated on clocks A and B. If the speed of light is indeed same in both directions, both signals will arrive at C at exactly the same time. If there were any differences, we could easily calculate speed of light in either direction ]]>