(it has to be via USB!)
Why? You can connect the serial port pins, and ground, between two Arduinos and easily exchange serial data.
Pushing the data through the USB to serial chip is a lot harder.
I have found alot information about wireing two boards connecting some pins - an option I can't use.
Why can't you?
But I have not found any information on connecting via USB - can someone point me to a guide please?
You'll need to write a driver for the Arduino to use to command the host shield. Not a trivial task.