Notch/slot antennas on commercial aircraft are single, fed by two transceivers and two tuners/couplers wired in parallel to that single antenna.
They are much more efficient, as they excite the entire airplane into an antenna, vs. just a probe. That's the reason so many 727 and some 747 were converted from probe to notch antennas, and why the DC10-11 have superior HF performance.
I don't understand all I know about it, but the 757/767 and other Boeing HF is often lacking. Boeing just hasn't understood and applied the principles, such as total resistance, which must be less than ten milli-ohms. I've had the good fortune to know Ben Hornby, ex-pat Brit at McDouglas, who was responsible for the DC-10 HF antenna, and some Brit plane before that, possibly the Trident.
There also must be as much structure as possible above the antenna, as in the DC-10, where the antenna is just below the number two engine, and the 727 with its T-tail. Other a/c aren't so well endowed for HF.
After the AA A300 accident, the awfully high currents involved set me to wondering about HF antennas in carbon fiber vertical fins, and possible long term deterioriation of the structure.
GB