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Two or more synchronized transmitters operating on the same frequency will produce nulls at certain points where the signals overlap. The position of these nulls can be controlled to an extent but they will still be there.
An alternative is to operate the transmitters on different frequencies. This may not be convenient for mobile listeners but would work for those who are stationary. It would be a matter for the listener to find the frequency which gives the best reception such as is done with FM translators.