For Atheros Chipset, there are five option of antenna selection.
Both; A only; B only; A-tx B-rx; A-rx B-tx.
if i select A-tx B-rx, will both antennas work simultaneously?
Or they switch between each other.

It will (should ) do what it is labeled like:
Transmit using antenna A, receive via antenna B.
So you could for example use a high gain antenna for receiving and a lower gain antenne for transmitting (as the higher gain antenna also used for transmission might violate your local regulations...).