I believe the intention is that it can be used for 2.4G or 5.6 (802.11a/b/g) ... i.e., the antenna is resonant for either band (versus optimized for one band or the other).
The docs (http://www.cisco.com/en/US/docs/routers/access/wireless/hardware/notes/antwall.html )mention that it can also be used in tandem for diversity.
If you want to use this model of antenna for each of two bands installed on your AP, then you would need at least one antenna for each radio (minimum two antennas, maximum four).
Good Luck
Scott