The Aironet 1200 has two antenna connectors, but you can use a single antenna. Just use the right antenna connector, which is labeled “primary”.
There are many advantages to having two antennas, because the access point supports using two antennas to improve signal through diversity.
The gist of the article is, you should use two antennas of the same type. This will maximize coverage for one access point. If you need to expand coverage beyond that, add another access point.