Hello,
Your assumption that RUTX12 and RUTX14 do have 2 antenna connectors for each modem is correct.
You can use an omnidirectional and directional Poynting antenna at the same moment too, but be aware that each modem does have 1x MAIN and 1x AUX connector and each antenna will be connected to a different modem, which means that if your second set of antennas will have better signal quality, it will affect only modem 2.
Regards.