The RW-12D has a lower port tune. Running both subs will eliminate the lower frequency reproduction ability of the RW-12D, because the BIC F12 will basically be running out of phase below its port tune - which negates the RW-12D's sound at those frequencies.
So yes you'll get more volume with two, but you'll actually have less low frequency sound than if you ran the RW-12D alone. Meaning say about 22-32hz will be negated on the RW-12D --- which really just negates the reason to have bought an RW-12D in the first place. Sell the F12 and buy another RW-12D if you want more spl.