If you actually see problems, you could get this LNA which has very good out of band attenuation:
New Product: RTL-SDR Blog 1090 MHz ADS-B LNA
The FA filter could improve things as well but i somehow doubt the attenuation would be enough with such a strong signal.
Mounting the antennas in each others dead spot should also help.
As you said you don’t transmit often on that antenna, so maybe it is acceptable to have no reception while transmitting?