I would not suggest using the BOTH position to start your car, ever, unless it won't start any other way. Even then, it is not how the switch is designed to work.
An overwhelming number of people seem to have the misconception that using the BOTH position on the switch is like jump-starting your car, but it really is not, unless both batteries are fully charged and at equilibrium with each other, which is rarely the case. If it WERE the case, one battery or the other should start the car just fine, you shouldn't need two fully charged batteries to start your car. If you do need two, something is wrong.
Here is why: think about what you do when you jump start a car with another car. You always have the other car running, and by hooking up the jumper cables you are essentially hooking up the dead battery to the good cars battery AND charging system. You usually won't get anywhere by hooking up a dead battery in a car to another battery all by itself. You might get a couple extra cranks, but it probably won't start the car. Hooking a good battery by itself to a dead battery will only cause the voltage in the good battery to drop, and continue to drop until the batteries both have equal voltage, probably dropping enough that neither battery will be able to start the car. You need the charging system to start a car with a dead battery, but it is not available to you in the car with the dead battery because the car needs to be running for the charging system to work.
Back to the Cole-Hersee switch...setting it to the BOTH position is essentially the same as trying to jump start a car with just a second battery, as I described above. You are essentially doing the same thing, you just happen to have both batteries in the same car and already wired together, but with a switch in between.
The way these switches are intended to work is to keep different types of loads separate from each other. You wire your starter and other normal things (radio, dome light, headlights, cigarette lighter) to battery 1. You wire all the things that draw current when the vehicle is not running (emergency lights, scene lights, etc.) to battery two. When you park and leave all your emergency equipment on, they will only draw from battery two, and battery one will be available to start your vehicle with a full charge when you need it.
When the vehicle is running, and the alternator/generator is available to charge a battery, you set the switch to whichever battery you want to charge, or BOTH if you want to charge both. When you are driving you would want it set to either position 1 or set to BOTH, so that your charging system is part of the circuit while you are driving. When you are not using the vehicle at all, you would set it to position OFF, to disconnect everything in the vehicle from the batteries, so a light or other device left on will not discharge the batteries.
This page is for boats, but it kind of describes the same thing. The top portion (2 batteries, 1 motor) is similar to what we have in ambulances.
