How to calculatevol.- with braces and sub

When building a box for two subs (enclosed) JL 12w3v2  x 2 . How do I include the dissplacement of bracing and sub housing?  How do I measure this? I.E.>sub housing equals an extra inch in box length, height, or depth?  Also when building one box for two subs should you include a partion between them? If you dont then wont it be like haveing twice the air space for one sub? Or  will two subs counter act  each others air volume, and act like one?

ps- Does it show that im a newbie? Sorry if this is to basic or simple for most, but I don't want to wast money or get frustated buy screwing something up because I didn't ask.

Thanks again for any help!!! 

Thanks for the tutorial. It really helped!!