To avoid the notches in the spectrum an SFN offset delay should be introduced in one of two transmitters.
This could move the problem to another location if the delay is
relative small (3us…5us…)
The delay should guarantee a reliable reception which will happen at the distance where the amplitude difference between two RX signals are relative large. If possible this distance should be set outside the overlap area.
Based on the propagation curves defined in the ITU Recommendation ITU-R P.370-7 (see Annex) it is possible to determine the distance and using the formula:
Delay [us] = (Distance [km] / Speed of light [km])*10
to calculate appropriate SFN offset delay.