Two of these will only need about 1.3-1.4 cft. ported (same as, or less than most single decent 12's).
And yes, two with 300w each are 3dbs louder than one with 600w... 3dbs doesn't seem like a lot, but it will sound twice as loud in the car.
FWIW, I like the idea of upgrading to the pdx 1.1000 later, though.