Mounting the batteries one on top of the other, has no bearing on the noise generated by the ignition. the ignition battery is isolated from ignition noise at the switch. Batteries make, nor do they transmit ignition noise. That's why it's important to keep the ignition switch as close to the ignition module as possible, and the receiver switch mounted as far from the ignition switch as possible. You can use the batteries anywhere you desire to achieve the proper balance.