Having two antennas on a transmitter makes no sense. Hooking antennas together in parallel creates a composite, screwed up radiation pattern with nulls and lobes.
Multiple antennas work on an airplane because there are mutliple receivers and a computer to decide which antenna to listen to at any particular time based on which is providing the better signal.
In order to use two transmit antennas to advantage the transmitter would need to transmit two separate signals (codes) with the airborne receivers/computer deciding which one to use. I don't think our radio systems do this.
Maybe the second antenna on the transmitter is the telemetry receive antenna?