well, the dual drive system is two sealed 15" subs, with two 1000 watt amps. Not just the size of the driver matters, but its specs as well. some 15 inch drivers only have 3mm of xmax while others have 53mm of excursion
These are major differences!! Bottom line, if you were to get two of the dayton 18" ho's and built two identical boxes to the hsu's only a little bigger, and gave each sub around 1000 watts, you would still blow that HSU out of the water, and trim the AT LEAST in half!!!! I would say at that point to not purchase a plate amp that I previously linked, but a standalone professional amplifier like the EP4000. It would power two just fine
you could also go for a 15" variety if you have space constraints, which at this point I would suggest the Stereo Integrity 15ht.
EDIT: Just saw Mike beat me to it