I have one 12" fi ssd in a 2.5 ft^3 box, tuned ~32 hz. Im planning on getting the sundown 1200d v.2 with another 12" ssd. I'm thinking about attaching the new sub to the front of the current one, (so the cones are facing eachother), granted they'd be out of phase and far enough apart so the surrounds dont make contact. Im wondering if this will produce the same or worse sound quality than I'd get with a regular 5.0 ft^3 box for the 2 subs. This way, I could save what little trunk space I have left without having to make a whole new box. Should look something like this:

If you have the space, just do a normal ported box. Isobaric is really only for if you are really cramped for space. It essentially doubles the power handling of your one sub. However, the increased cone area of actually using the two subs separate generally gives better results if you have the room because you can use the advantages of both double power handling and cone area.

They should sound the same, however. Maybe you should try a sealed box?

actually adding another one on your current enclosure will change the bass response drastically.

You will essentially have a single 12 SSD in a 5 cu ft enclosure.

Isobaric loading halves the Vas...making the enclosure seem twice as big to the driver

Would doing the iso set up with one sub thats a year old and a brand new one cause problems if they have different excursions during the time it takes to "break in" the new sub even if they're not hitting eachother?