If you are serious about doing it this way, you could eliminate the oil pressure switch and just wire it with the self-start override as your in-dash switch. When you flip the switch, you'll have both batteries. Here's a question, however: Why do you want to rely upon yourself? The oil pressure switch is idiot proof. And with the solenoid and all, you have both batteries linked together. If you have this switch only setup, you are readying yourself for a problem. If your stereo and extra lights are wired to this second battery, and you flip them on without turning on the second battery switch, you'll drain the second battery...dead. On the other hand, leave the vehicle off/switch on long enough, and the unisolated batteries will kill each other. Leave the switch on/vehicle off/headlights on, and you'll come find your jeep with two dead batteries. A lot of folks have put a lot of thought into this one. If you're running enough extra lights and stereo to need a second battery to keep it going, all you're going to accomplish is double the amount of time it takes to kill your battery. Batteries are intended as reserves...storage for peak loads, like starting and winching (and welding/wwwthreads_images/icons/smile.gif). If you're going to be running heavy constant loads like a stereo or lights, you'd be better off upgrading your alternator. Don't get me wrong, I'd like to have the dual batteries anyhow (nice to be able to jump-start yourself when you leave the lights on), but you might want to reconsider a bit.
Evolution of tools: stone, hammer, wrench, socket, impact, really big hammer, blue wrench.