Can somebody please tell me what the purpose of 2 antennas are for on wireless adapters over other wireless adapters?
Depending on the adapter, there are sometimes different purposes for multiple antennas. The most common reason is to support diversity, that is, the antennas will switch between one or the other, whichever may find the best/strongest signal. This is done in an attempt to eliminate signal fading or dead spots. A quick tip, it’s been found that the best way to position two external antennas is perpendicular. That is, one straight up and one horizontally, as this can improve communication with client wireless adapters.
In the case of some adapters, the antennas support MIMO (multiple input, multiple output), so they can both work together at the same time. This allows for more data to be transferred at the same time, as well as an increase in the range because they can combine signals from different paths and at different times to increase the reception. You find this more often in the newer adapters and antennas.
Dual antennas could also work to broadcast the signal from different bands. The two most common bands in wireless networking are 2.4 Ghz and 5 Ghz. Sometimes, the two antennas will each send the signal for each band, but this is not always the case. For the non-technical users, this is almost impossible to determine if this is the case.