Ignition problem?

My truck has 8 plugs and 2 coils. One coil works the other one will not. I switched the coils and the coil that does not work, works on the circuit that the coil does work. Therefore, I am sure that the coil is good. In addition, when the gas pedal is pressed all the way, the coil that usually works stops and the engine dies. I checked the coil wire with a tester to make sure that it was not firing and when the gas pedal was pressed all the way down it stopped firing. Could it be a relay or a switch that is causing the problem?