Hi again,
I spent some time thinking for a way to synchronise my arduinos. I've got an idea and I'll be glad if you tell whether it is possible or not.
Since all devices on the i2c bus share the same SCL line and only the master controls the clock, would it be possible on each arduini to read regilalrly the SCL line and to increment a variable every falling edge for example and so all devices shold have the same clock.