I just hacked together a platform similar to this.
for the back of my truck to fit in the amps. (Rear seats were pointless to me.) I'll have to finish the box and then mount everything before putting it in the truck. (It'll be loads easier.) I'm also running 4 ga. cable from the battery to the rear to power it all (two amp system) and allow for future upgrades. I'll probably have to vent the box as well to get some air flow so the amps don't overheat. (Couple of fans might not be a bad idea either....)
A second amp for the 4 channels would enable you (with most amps these days) to tune what goes to your speakers so they're not maxxeed out by bass and you get a better sound. Likewise, the volume on the h/u itself won't have to be turned up as much, ergo, less distortion and cleaner sound. It's worth it to me to add it in just for the overall cleaner sound. Besides, most aftermarket speakers need more power to really bring out the sound. My rockford 6x8's are currently hooked directly to my h/u and they sound ok, but it will be MUCH better when I add in the 4 ch amp.
My suggestion as far as mounting would be to mount both amps in the same general area so that running power and rca cables won't be such a big pain. Run your rca cables from the h/u to the amp, then run a wire set back to the original stock harness and tap into the adapter you have. This will allow you to return everything to stock in a short while and keep you from having to run wire all over the place to the four corners.
Take your time and plan everything out. Draw up a plan for what you want to do so you can visualize it before you start building it.
Hope this helps.