You can install 1 or 2 antennas. When you use 2 antennas the AP needs to be set up as using diversity antennas. This is usually the default. If you use only one set it up to use just the 1 antenna. Diversity antennas gives the reciever 2 reference points of signal to enable it to get a good recovered signal in the case of multi-path or delay spread on the signals reaching the AP.
