The easiest way to measure the synchronization of the cameras involved using my turntable with a corresponding scale:
Setting the turntable to 45rmp means that a deviation of 10 degrees corresponds to a time-difference of 0,037 seconds.
Here is the first test shot:
The time-difference is aproximately 0,015 seconds although the second camera has received some flash from the first.
What you also see is a significant deviation in the vertical direction, which means I have to adjust the rig accordingly. Time to work on the next post ... ;-)