Wow, that is a huge antenna. The specs say 6' 7" length, but it sure looks a LOT longer than that. Otherwise, the rest of the specs look great. Never can remember seeing an FM only antenna coming even close to 12 db of gain. And yes, it is a bit pricey (ouch)
You may want to visit the
Digital Home site thread on stacking antennas to answer questions about stacking. You can generally expect a 2 to 2.5 db increase in gain when stacking 2 identical antennas.