Since a picture is worth 1000 words, here is a simplifed graph that shows, as a first approximation, how an aircraft's VHF radio range inceases with altitude.
The "Line of sight" radio range between any two objects, is the sum of their individual "Line of Sight" radio range, for their given altitudes.
( Ignoring terrain between the two objects, that can reduce that range)
Referring to the graph below.
If a Plane is at 1000 ft, it would have a range of about 30 miles to an object on the Ground,
If another plane was at 10,000 ft, it would have a range of about 100 miles to an object on the ground.
The radio range between the two planes would be 30 + 100 = 130 miles
