The two connectors can NOT be used for Dual-Nstreme, if you mean that. For that you really need two cards, because a wireless card can only send OR receive at one point in time, not both the same time.
What you CAN do, is connect two antennas (one to port A, one to port B) and configure RouterOS to use one antenna only for sending, and the other only for receiving.
One possible usage would be to be able to use a high gain receive antenna without it pushing your transmit power level over regulatory limits in your area. I.e. send with a 14 dBi antenna, and receive with a big 30 dBi ear