Hi Greg; Q1,
Are two woofers better than one? Hell yeah! If ones good, two's got to be better right? ........ Cut the required excursion and you cut the distortion. Q1a, Will two 12's "bring it" better than one 15"? Depends on what bring it means to you.
To me? I like well controlled drivers so if you compare apples to apples,,. Lets use 2206's, 2226's and 2241's here, the common part of these transducers is the motor assemblies, they are identical. If you do the math a pair of 15's have about 63 more inches of cone area than a single 18, and fully twice the motor force power. Which do you think will have more punch, or pop, or snap? ........
Q3, Two woofers in one cabinet, separated or not?
For me this depends on how high you are going to use them. You need not worry about standing waves until the enclosure becomes acoustically large ie one half a wavelength at the highest frequency of interest. A sub enclosure crossed out at 80Hz needs no thought given to its internal dimensions until one of them reaches right about seven feet. This is a very nice gift from the science of small room acoustics, a degree of design freedom that is rare indeed in the world of acoustics. The problem comes at you fast however as at 1000Hz you only get about 6.75" so at some point you are going to have to deal with internal resonances. If I am going to run a pair of woofers up fairly high, I split the box with a divider, no ports,......
On the cabinet size vs driver thing, yeah you can put a driver like a 2241 in an oversize box and port tune it low and EQ it flat, but I have never been able to get them to sound really good doing that, that why JBL made 2245's. It has become apparent to me through much, much, much trial and testing that the more you have to force a design, the harder it is to get great sound form it. Use the right drivers for the job. I am all about taking the path that leads to knowledge. When you step out and do, you gain knowledge and experience and that first hand knowledge is power. But realize this, that even though you can surely figure this all out on your own, a good deal of time can be saved by searching and understanding successful designs that already exist. JBL has done a lot of this homework for you.
Oh yeah, never listen to anyone on the internet! Barry.