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You might consider the bandwidth of the FM signal. Usually, when modulation percentage is consistently averaging below 50 percent, the occupied band-space of the signal is so narrow that older analog receivers, with weak AFC circuits, have difficulty tuning the center of the signal. Stereo makes the modulation level of the base band signal even more critical on some receivers. Of course, a low RF signal can manifest itself as “hard to tune” as well. Be aware that two antennas in reasonably close proximity to each other will have signals that phase together at some point creating nulls and lobes in the signal pattern. This may result in “picket-fencing” and distortion in the receiver.
The lengths of the two separate transmission lines (beyond the the “Y” or “T” connector) will also cause impedance transformation and phase delay for the signals reaching one or the other of the antennas.