I'm not quite sure why you think that you need two arduinos to accomplish what you need, but you know your system better than we do.
Is your question then, how to turn on an LED from one or both of the arduinos at the same time? If so, I literally just answered a similar question in another thread. You can drive them with open-collector transistors. So either arduino can turn the LED on, but they both need to agree to turn it off.
Depending on how much current the LED needs, though, you may be able to drive it directly from two pins on two different arduinos without anything extra besides a pair of resistors.