ok first of all its not absolutely nessicary to have seperate chambers for 2 subs. when i get my eD 12O's, i'm doing a common chamber in a 3cuft box. its actually easier that way because you dont need to calculate exactly where to have the divider for the chambers, because if you're off by a half inch or so, guess what? you're volumes for each chamber are going to be different.
its also better to do it this way esepecially for ported boxes because you don't have to make two ports and try to tune them exactly the same. if you do it with a divider and try to tune two different ports exactly the same and you're off by say half an inch again on the port, guess what? you have the same two subs playing the same frequencies, but since they're at two different tuning frequencies because of that half inch, you're most likely gonna get some cancelation. so its actually better to do a common chamber do a sum of the volumes/port volumes so there is absolutely no mistuning of your enclosure


secondly, whoever said you have to figure out the proper volume for a box if its sealed/ported, well thats kind of obvious. but proper is in the eye of the beholder, depending on what kind of sound/output you want from your system
you also need to know your tuning frequency as well when you have a ported box

third, you don't ALWAYS have to build boxes to manufacturer's specefications. don't be afraid to experiment.
do you think SPL competitors acheive huge numbers with 4 15s by adding up the recommended sealed or ported volume and tune it to like 28hz or something low (if ported)?
no, they make enclosures with bigger volumes, tuned higher because that will typically increase output.

typically a manufacturer's enclosure blueprint is a good recommended volume/tuning frequency, but you don't always have to go by these standards. it all depends on what kind of sound and output you want from your driver