We have speed of light = 299792458 m/sTo have a accuracy of 10% the light has to travel 1/10th of a second between the observers so,Distance travelled by the light in 0.1 s = 0.1×299792458= 29979 km.The difficulty in separation of that distance will be the curvature of earth. As the earth’s surface is curved, light from one of the experimenters won’t reach the other.