Not much to see except for the JL logo. I looked for shots but nothing that will give you a good idea of what was done.
Yes, two subs are impossible, both from an excursion perspective and airspace. We tried, loving the idea of using both grills for speakers. But there's just not enough room behind that back panel. But one sub fit ok with a few issues.
We had issues with airspace. So we cut away the metal panel behind the seats so that a custom box could be built slightly larger than the factory plastic ported sub enclosure. We now had all the airspace we needed. The JL sub I chose fit in the box perfectly and actually touched the back of the box. To eliminate reverberation, I dynamatted the back of the box and physically attached the sub to the back of the box. We also built it as a sealed enclosure rather than a band pass like the stock because that requires less air space (I had about 1.5 cubic in the box).
To get the most space, we used dense, but thin wood. To gain a deep sound affect we dynamatted the entire outside of the box and used stuffing on the inside.
Excursion was our last problem. The speaker would sometimes hit the inside of the cap that goes over the box. We simply altered the back panel not to slide so far back behind the seats. This gave the speaker room to move without hitting the speaker grill, we only needed 1/2 extra. No one ever noticed what we did, still looked fairly stock.
The thing I kept in my mind is that I wasn't looking for tremendous volume from the center speaker. It'll over power your other speakers simply because it's inches from your ears, staging wouldn't be correct. Because of the changes I made with the speakers and amps on the car, I could begin using the center speaker as a woofer or subwoofer rather than a mid-bass as the stock was tuned. So a bigger center speaker, running at lower power ranges, gave a nice deep sound with little strain - while the other speakers in the car were of higher quality, bigger amps, running at lower frequencies to create the sound range and volume I needed.
I had to compensate for that loud Belanger headers and exhaust! At least that's how I justified the money I invested. :.)
Hope that helps.