Sync 12 Arduinos - I2C vs Serial
Use two pins per chip an input and an output and chain them together serially. Have the "master" chip drive the toggle the pin each 'frame' and all the slaves interrupt on pin change, register the heartbeat event and 'sync', then propagate the pin state to the next chip, and so on... if you want to get fancy, wrap the output of the last chip in the chain back to the master so that the master can determine whether the message was received and what the latency of that response was.

This is a nice, simple approach that doesn't require a lot of software sophistication or much worry about electrical characteristics and signal integrity. There are, obviously, other more complex and feature rich approaches to solving the problem you pose.