The AP-228 has discreet radio antennas (not dual-band like many other external antenna models), so just using 2 omni-directional antennas will either reduce the radio performance to a 1x1:1 on both bands, or will eliminate one band and provide 2x2:2 on the other. Regardless, performance will be less than if all antennas where connected/used.
If you leave antenna ports disconnected, remember to add terminators on the unused ports.