What AP model are you planning to use? If the AP-228, then you can use separate 2.4 and 5Ghz antennas since those antenna connectors are not diplexed. However, using your numbers, to generate a cell for an approx 75x50m area at 15m AGL, 30deg would not be enough beamwidth, you would need ~60-90 deg min.
Assume radius of 35m (to cover approx 70x70 for easy math), at 15m height, drawing a right triangle, that angle at the top would be approx 60deg, for a 120deg beamwidth need. Using a 60deg or 90deg antenna would be determined by your spacing/overlap ratio plans. But 30deg seems kind of narrow for such a small cell at such a short height. A 30deg antenna aimed straight down at 15m elevation would give a coverage of 4m radius or 8m diameter. That's too small.