Solution

The last point they would be able to see each other is when the line of sight is tangent to the Earth's surface as in the picture.

The distance we want is the length of the green arc between the two people. (Not the line of sight)

The first step is to find out what the angle A is. (Actually we will need to know what 2A is...)

So now we know the angle, need to use this to find the arc's length ():

Converting degrees to radians is analogous to a unit conversion:

So...