You will typically need double then... I have nener used teh subs and have no clue about there parameters, but if they say 6 cu/ft for one it would be 12 cu/ft for 2.
i know that its just going be the tunning for 2,if shit hits the fan i prolly just do 1 6cubes 48hz for both apps but i dont know cuz it going be my all around sub