don't doubt the quality of then sundown sa 12s, I personally thing they are the best 2.5 inch coil sized 12
1 18 has more cone area, then 2 12s. so get that right first.
cone area = pi x ( r x r )
r= radius
pi=3.14
1 18 has about 254.47" of cone
2 12s has about 226.19" of cone
if your going for numbers 1 18 is proven to put out better numbers then 2 12s. taking into fact which subwoofers are used
you will probably get better numbers with the ns 18 over 2 sa 12s with the right power
however the 12s will be more musical, and require less room