Curious about this as well since I have a spare twin to my sub sitting on a shelf collecting dust.
So if you build a single box to house 2 subs how do you figure out the size/volume requirement? I am currently running a 12" Polk in a sealed box. Some songs give too much bass, others I'm looking for more. In the end I'd rather have more than less. I have a ton of room under my helm - enough I believe that I could build a bigger sealed box and have both subs in it - most likely one facing up, the other facing back. For example if each sub suggests .75 cu ft for a sealed box then would I be looking at building a 1.5cu ft box? Running them parallel for a 2ohm load I'm confident I have plenty of power off my sub amp still. Just wondering if it would be worth it I guess.
You have enough room for a family of immigrants under your dash, throw that second sub under there! We can throw a box together in a hurry. Are they the same subs?