running 2 amps and spare battery

Get the second amp with output jacks, and you'll use RCA's to connect it to the other amp.  No splicing of wires.  There will be a full range signal to both amps.  Post your car & year, system makes and models to get the proper recommendations of your car's necessary power upgrades.

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Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.

And adding a second battery and cap may not be a solid idea, you still have to charge those babies.

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Top Secret, I can tell you but then my wife will kill me.

If you are gonna get a second battery and cap, then put the cap under the hood or right near the firewall inside the vehicle and then the second battery in the trunk. By doing this it stabilizes the voltage running through the system, kinda like using a line driver on the preamp lines.

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double-secret reverse-osmosis speaker-cone-induced high-level interference distortion, Its a killer

A second battery will only help for playing your system without the car running. It wont necessarily give you more power when the car is actually running.

Adding a second battery to a audio system isn't as simple as most people would have you believe. To do it safely and properly you need the proper equipment. Unless you plan to play your system for long periods with the engine off, don't bother. Adding a capacitor would be a better idea. Your stereo system gets all its power from the alternator when the engine is running. The battery's job is only to supply the starter motor with enough current to start the engine. After that the battery becomes a load on the alt. as it recharges. The battery will also filter and regulate the power to the car and act as a reserve under high load conditions (everything on at idle0.   The battery is a capacitor, but it is designed to provide a huge amount of current over a few seconds. A audio capacitor is designed to discharge a large burst of current instantaneously to an amplifier. Amps require these bursts for low and loud bass notes. The cap should be mounted as close to amp as possible with the proper size cables to coduct the current effeciently. Get a one Farad capacitor for every 1,000 watts of amp power. As music is dynamic your amps won't always requuire huge amounts of current all the time. Adding a cap will help your amps with their ability to deliver great bass. As for your signal lines, you can purchase "Y" adaptors that properly split an RCA signal into to lines. Make sure you get the correct ones as they come in two forms. Two males to one female or Two females to one male.

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sparky

definately get a cap. dont put it by the firewall, that's pointless. the caps gain would be lost to the distance. put it as close to the amp as you can get it. if you want a second battery, that helps. but you need to get a battery isolater. or the batteries will draw off of each other and you will always have 2 dead batteries. i have a 1600 watt amp 1200rms MOSFET and i use all of it off of a stock alternator and a 1 farad cap. even when turned all of the way up, my headlights don't dim.