dual batteries?

[lo-lyf]

Wire the batteries in parallel. + to +, - to - to ground.

That's an easy way, or if you want to keep one isolated as a backup you can wire in an isolator relay. Say you spend a lot of time running your radio and want to be sure you can start your truck afterwards. Wire the relay to be controlled by the main ignition(pink wire, green plug at bcm, stays on during crank). Either way works, just depends on your needs.