I would give up this solution if found a way to make different arduinos synchronised : start exactly at the same time and have
the same clock, or parse files at the same speed with no shift between them.
I gave you a solution for this, just use one digital I/O to synchronize the Arduinos. As you haven't described what kind of system you're trying to simulate it's not possible for us to help you with more details or even suggest a different way.