> I'm thinking about one camera being the "master" and sending its shooting settings to the "slave"
This has been done.
The author has been occupied with other business for the last four weeks but should resume soon.
The master camera blinks the blue led to communicate with an ATTiny PIC.
The PIC then sends the settings to the slave using USB.
The cameras can be highly synchronised.
The code will be made freely available and ported to SDM and a branch of CHDK.
It is not known how to communicate via USB.
EWAVR did play around with turning one of the USB data lines high and low but there were various restrictions, I cannot remember what.
David