If you are combining two 2x2 antennas (i.e. you have 4 cables total to connect) you don’t need to use that option, that is for if you ONLY have one 2x2 antenna available (i.e. only 2 cables total to connect).
How best to connect them might depend a little bit on exactly what the antennas are (perhaps tell us exactly what the antenna make/model is) because how the antennas are polarised and how they are mounted might influence that decision.
If you have an antenna that is say 2x2 XPOL then you could just do:
Antenna 1 = A+D
Antenna 2 = B+C
If you have for example 2x vertically polarised antennas then you might get improved results as @bryn.loftus mentions by mounting the antennas in a way to create some more polarisation diversity, in which case try:
Antenna 1 = A+B
Antenna 2 = C+D
In any case for reference A+D are TX/RX ports, B+C are RX only.