Perhaps this thread can be of some help: Dual GPS setups
If I understand correctly, the dual F9P setup sends the heading and roll information over UBX protocol, not NMEA. Only the position from the first receiver is sent over NMEA. That’s why a microcontroller is used to process the UBX data and create a custom NMEA message for AgIO to use.
Alternatively, AgIO and AOG can directly process NMEA from the SkyTraq PX1172RH dual antenna receiver board, which isn’t much more costly than the two F9Ps, just for future reference.