Set a single boolean control to be either "Switched when pressed" or "switch when released" These change state only when the you press them or release them (I prefer release because it gives you the chance to drag your mouse off the button in the event you want to cancel yourself mid-press). The true state of the button represents one voltage, the false state the other. Then you can feed the boolean into a case structure or boolean comparison selector as the case may be.
As for reading 1 time, it depends on what you mean by that. If you only want to take action only when the value changes, you may need to add code to your loop so that it compares the current value to the previous value and only does an action by way of a case structure when the values are different.
