1) No problem, the closer to the floor, the better.
2) Here's the way I would equate this one. Lets' look at the size of the pistons involved, 9.7" for the DD-12 and 15.2" for the DD-18. Using (pi r**2) to get the area for each, we come up with 73.9" for the DD-12 and 181.4" for the DD-18. So if we add the two DD-12's that would give us 147.8", well short of the piston size of the DD-18. The throw (push) of the pistons would be constant in this example, so you can't double up on that figure. So two DD-12's would be closer to the output of a single DD-15 (126.7") than the DD-18.
In general, a second subwoofer of the same size and type, placed in a supportive position will add about 6db output.
The decision of two small subs vs one large one often has to do with placement space or attempting to get flatter frequency response at the listening position.
I hope this helps some.