For a single subwoofer ported system, the maker's literature reccomends a 1 cubic foot enclosure with a port 3"id x 15".

If I want to build an enclosure where two identical subwoofers share the same chamber, is it easy as doubling the enclosure volume and adding an additional port of equal dimensions? (2 cubic feet and two ports, each 3"id x 15")

That seems reasonable to me, but I'm no expert. I know that port volume makes no difference, and it's the relation between the cross-section and the length of the port that determines tuning.... because of that I question my logic. Thanks for any input.

 

www.jlaudio.com/tutorials/

 

I've been through those but could not find any information on ported enclosure calculations using more than one woofer in a shared chamber. Thanks anyhow, those tutorials do have loads of great info. Maybe I just overlooked something.

 

have you thought about having two chambers next to each other instead of one shared chamber? in my opinion it would be easier and sound better

 

In the old days of ported enclosures we were told that two drivers would not share a port or ports equally. But I think science and common sense has disolved that theory. A box is a small pressure chamber. As such the sound, SPL, etc. must be considered as pressure variations. Maybe in the old tests they didn't have each port placed in a way that the ends were not obstructed or possibly they didn't know about end correction factors or other tuning variables.

JL audio definitly has the info you are looking for.
http://jlaudio.com/tutorials/ports/index.html

Another valuable resource for enclosure construction is WinISD Pro alpha, http://www.linearteam.dk/