no it's never easy ;-)
What about this one:
Create a small circuit of a resistor of 1K, phototransistor, power supply and DSO digital oscilliscope.
(see attached image)
Then switch on the flash on both S110s (lowest power of flash will be enough).
Then take a photo using USB switch, sync enabled.
Then both cameras will take a photo and fire the flash. The DSO should detect two peaks from 5V to 0V, one from the flash of the first cam, the other one from the 2nd cam.
The closer both peaks are together (on the x-axis of the DSO) the better the cams are synced.
The sync delay can be calculated by the time difference of both peaks.
Can/will that work?