I am trying to figure out if I should go two JL 12w7's and a 2k watt amp or something like a DD 9512 in a big box? Which is going to hit a higher SPL?

 

That's a VERY difficult question to answer.
1) Either way you're overpowering the sub(s) wuth 2KW so caution will be the word
2) the JLs would have twice the cone area so it's not apples to apples to begin with.
Assuming either box is well built for the sub(s) you choose I'd say the JLs would be louder purely on cone area.
BUT I bet they cost more than the DD's- why not buy 2 2512s and rule the block??

 

Go with the DD 2512s !

 

Between the two it depends on many things - the box they're in for one. How much cubic feet is available to you? Do you have the cubic feet requirements for two subs or would it be better to put extra cubic footage to the one sub?

I had a guy come in and do a $25000 stereo which was four W7's. He wanted to compete but still wanted it to sound good - lol. Anyways, about a year later two of the W7's broke down. Funny thing was is the subs got louder -and that's because the two subs had more space available to them and was more suited for the subs. This was in the back of a Ford Focus too, so he had plenty of room. But not enough for four of these monsters.

So really it comes down to your box. Don't go with two if you're sacrificing space, go with one and get a nice box built for it. However if you have the space requirements for two definitely go that way! Take your time with it too, don't rush anything because the time spent will be worth it.