Add the velocities of the two objects together to get their total velocity. For example, if you know that one Boeing 747, label it "A," is moving at 100 meters per second and the other, "B," is moving at 200 meters per second, you can add them as follows: 100 + 200 = 300 meters per second.
Multiply the two speeds together, divide them by the speed of light and add one to the value you come up with. The speed of light is 300,000,000 meters per second. Complete the calculation as follows: (100 x 200) / 300,000,000 = 20,000 / 300,000,000 = .0000667. Adding one to this gives you 1.0000667.
Divide your answer from Step 1 by your answer for Step 2. For the example calculation, this is 300 / 1.0000667 = 299.97 meters per second. The planes are moving at 299.97 meters per second relative to one another.