Yes, if one sub is 1 cuft, then two subs is 2 cuft (1 cuft/sub). Generally you'll want to make individual chambers for each sub, that way if one blows, the other doesn't instantly have double the volume it should. Good precaution. You can just make a box that has a spacer in the middle to seperate the two chambers. Remember that the spacer in the middle takes up airspace, so you have to factor that into your internal air space.
Thats what I have, If you go onto the bcae site, and use the calculator you should have no problems. I had a space issue due to mine are in a single cab pickup, but they will definitly show you all the loose rattly stuff in your rig as soon as you get the volume up a bit.