Hi Hassan,
You can use two 1728 antennas in Diversity mode but not to cover two different cells/areas. For most installations, if you just used one 1728 antenna connected to the Right-hand/Primary, RP-TNC connector you should be good, if you have some obstacles in the cell area you can try using Diversity mode(2x1728 Antennas);
Cisco Aironet High Gain Omnidirectional Ceiling Mount Antenna (AIR-ANT1728)
From this doc;
http://www.cisco.com/en/US/products/hw/wireless/ps469/prod_installation_guide09186a0080286064.html
When you use a pair of antennas with matching characteristics to provide diversity for cell coverage in your facility, the guideline is to put those matched antennas at a distance apart from each other that is equal to a multiple of the wavelength of the frequency that is being transmitted. The 2.4 GHz wavelength is approximately 4.92 inches. Therefore, to support diversity on a 2.4 GHz radio with two separate antennas, the antennas should be spaced approximately 5 inches apart. The antenna pair could also be spaced at multiples of 5 inches, but the distance between should not exceed 4 multiples: reflected waves farther apart than that are likely to be so distorted and different in delay spread that the radio could not work with them.Because each antenna is selected by itself, both antennas must have the same radiation characteristics and be positioned to provide similar cell coverage. ***Two antennas connected to the same access point must not be used to cover two different cells.***
From this good doc;
Multipath and Diversity
http://www.cisco.com/en/US/tech/tk722/tk809/technologies_tech_note09186a008019f646.shtml
Hope this helps!
Rob
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