In your lumiere function, if AV > GA you delay,set the motors and delay some more. Only when the if is false do you check whether there is any serial input. Thus, if AV > GA on both arduinos, they will continually send 'H' which will never be read by the receiver. If the system runs in this state for a while and things change so that that if is false, when you finally do start reading the serial port, the buffer will be filled up with H's. Your delays ensure that it'll take quite a while to get through them all until you finally get the 'L' you're looking for.
I'm not clear what you're really trying to do though.