In this paper, relevant mathematical models and conditions for determination of obstruction shadowing in the Bullington double knife edge diffraction loss computation is presented. The paper introduced the concept of effective single knife edge equivalent clearance height, hse which is always greater or equal to the clearance height of each of the knife edge obstructions in the signal path. Sample numerical Bullington double knife edge diffraction obstructions are used to demonstrate the application of the mathematical models and conditions presented in this paper. The numerical example is for a 20 Km wireless communication link with dual knife edge obstructions h1= 30m and h2= 16 m where h1 is located at 2 km from the transmitter and (d2t) the distance of h2 from the transmitter is varied from 19 Km to 2 Km. The results show that obstruction 1 just overshadows obstruction 2 at d2t= 11 Km. At this point, the distance (d12) between the h1 and h2 is 9 Km. For all d2t> 11 Km, the obstruction 2 remained overshadowed by obstruction 1. When overshadowing occurs, the single knife edge equivalent clearance height, hs determined using the classical Bullington double knife edge obstructions model is less than the effective single knife edge equivalent clearance height, hse obtained. The corrected effective single knife edge equivalent clearance height ensures that the correct value of diffraction loss is obtained when dual knife edge diffraction loss is computed using the Bullington double knife edge diffraction loss model.