You calculate the volume of the box as a single chamber.
When designing a 2 common wall port like that you split the port in half at the back wall, so that half of the port area goes to each driver. Being that the port shares two walls with the shell of the enclosure you need to account for that (Torres will if you select it).
You can refer to this thread for some more info.
Thanks for the links, it did help some. I did come across another design that caught my eye, just to keep cargo from making contact with the subs, which the box by the way is going into a 2005 Ford Explorer. Heres what I'm working on. 4 - L7 10"s. The outer dimensions I would like the box is: 45"x12"x18" with the subs pointing up and one large port on the bottom (see pic). I got confused using Torres and the other links and I hope the end result meets the suggested specs.
The inner port measures 40"x9"x4".... I'm kinda leaning towards a sealed box for SQ. Just for reference, I've always built my boxes in individual chambers but have noticed boxes with one single chamber. Does it really even matter? Let me know what you guys think.Thanks...