The two antennas on the MAIN and AUX SMA ports will work together to increase data throughput.
It is recommended connecting both antennas to the SMA ports for better throughput. However, if that is not possible, a single antenna on the Cellular Main SMA connector is enough to connect to a LTE service.
MAIN antenna is used for transmit, and both MAIN and AUX antennas are used for receive.
To identify which connector is MAIN and AUX please check this link.