So for arguments sake if I have 2 12 inch subs that require 3 cubic feet pur sub. And all I have is a 3 cubic box that is made for 2 subs instead of one and put both of them in there would it be louder than having that same subwoofer in the box by itself rather than suffocating both of them
Maybe, depends on the sub. Here's three examples of entry level drivers with very different sound profiles that we are trying to build a box for to match the sound profile we want.
Kicker C12 4ohm ~ 300w
Polk Dxi 1240 DVC ~ 320w
Sundown E12 DVC ~ 500w
These subs are all the same size and similar ish power specs. Itd be easy to buy an amp that rocked all three.
The goal: A daily build with even response from 60hz to 30hz or deeper that hits into the mid to upper 130's (different subs could get louder in this hypothetical). Graphing in WinISD would show an even line or one with minimal raise (less than two db peak at resonant frequency).
Results: The kicker would need around 7 cubic feet of air tuned low to bring the low end back up to due the tight suspension and low excursion combined with low power handling. Use the port on the box to extend the frequency range and control the excursion of the sub. You'll want decent port area, but a little smaller than the others.
The polk would need 3 cubic feet tuned to 28hz with a good size port. I can tell you from experience that these subs will hit 25mm of excursion in a good box if you keep the air velocity in the port down.
The Sundown would need around 1.7 cubic feet tuned to 30 hz with a large port. The lower the air velocity in the port, the less noise and pressure your sub has to fight. This allows for more displaced air and a louder build.
Now, each sub will sound different in terms of distortion, transient response, and authority in the low mid bass range. However, they will all play 60 to 30 hz with authority and at relatively even levels. It'll sound good and clean (if the sub doesn't distort).
All three are 12 inch subs, all three were with in 200 watts power consumption of each other, and all three required very different builds to get similar results.
That being said, if you have a sub that is like the polk and wants three cubic feet of air and you slap two of them in 1.5 each, you'll loose your low end, over all volume (again, more pronounced the lower you go), and thermal power handling. Your subs won't be able to move as much and the coils won't be able to disapate the heat as well. Now, putting the box in your car will bring a lot of the lower bass back, how much and where depends on the car (typically around the low 40hz to upper 30hz range). A lot of people will build a box that drops the low end to let the vehicles cabin gain bring it back up for an even in car response to better blend with their mids and highs (an SQ setup). So it can be useful and sound **** good, but you'll loose volume overall doing it.