We have speed of light = 299792458 m/s
To 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.