If you are in a relatively strong signal area for both transmitters, using a less directional antenna may work. Even though the transmitters are in different directions if one has a strong signal in your area a lower gain directional antenna favoring the weaker transmitter may work.
If you need high gain in both directions, then combining two high-gain directional antennas using a "UHF splitter" backwards should work. There will be ~4.5db loss compared to either antenna alone.
If you cannot tolerate the 4.5db loss, then either single high-gain antenna and a rotator or two high gain antennas and a coaxial antenna relay is the only other way to go.