What would be best way to strap them together?
Side-by-side on an aluminium bar.
Use a miniUSB right-angle connector to reduce camera spacing.
Do not get nearer to subject than 4 metres and use standard focal-length or slight wide-angle.
how is flash synced?
SDM does that automatically using the cameras own flash.
One flash is fired at -4 stops so that you do not get double shadows.
It is possible that in a few weeks time a device will become available that allows you to use external flash units.