I don't understand this problem, I don't think. If it was so imperative that your generals attack at precisely the right time, why would you not have coordinated this attack long before the assault? Is that not a solution?
The imaginary Red Army, and the Imaginary Blue army were both being attacked by the Green army. After waging a long war the Red army and Blue army arrive at the capital of the Greens. The Red and Blue Armies have never met, and are only united by their hatred of the Greens.
The only way for the Greens to be defeated is for the Reds and Blues to attack on two fronts simultaneously.
My attempt at an explanation: At any given point, one general has less information than the other. When I send a message I learn that I've sent a message, but I don't know that they've received the message. When I receive a message, I learn both. If there is any specific threshold of knowledge required to go, one general must pass it first.