actually cone area of 2) 12s is close to 1) 18.
6" x 6" x pi x2= 226.08 sq inches for 2) 12s
9" x 9" x pi = 254.34 sq inches for 1) 18.
2) 12s can be louder but not in all cases.
2 subs will have an advantage because of the amount of voice coils. the power will be spread among more coils and heat will be more effectively dissipated.