I know I could do this using USB synch but I would need an external USB trigger system. Is there any way to use CHDK scripting to run an intervalometer and trigger the second camera via USB?
There is not simple way to just connect a cable between the two cameras and have it work. Your camera is a USB slave - it needs to talk to a USB host. Two slaves can't talk to each other - that's a hardware thing that can't be changed with software.
One way to do what you want (with a little bit of external hardware) is to use the AF-assist lamp on one camera, via a phototransistor to trigger the USB input on the other camera. Unfortunately, you can't buy such a device off the shelf (AFAIK - although it rings a bell that somebody sells something for KAP that might work).
