speed of 802.11n using 2 antennas
Multiple antennas on routers are used for beam-forming.  

Each antenna can handle whatever standards the router supports, but the more antennas the router has the better job it can do to form directional capability to reach each device that is connected.

I suggest you do some searches on Amazon and Google and read the specs for different routers that have various antenna configurations - they will describe this capability so they can justify higher prices for routers with more antennas.

Take a look at the Wikipedia article mentioned in comments to begin.

The more antennas the router has the more efficiently it can communicate due to the virtual beams, which can result in higher speeds.  

It is not because it uses different 802.11 standards on each antenna.