Arduino ADK, other board, shield or what?
You would need a USB host shield to achieve what you want.

The Arduino by itself is not capable of connecting to both a PC and another Arduino through USB without the addition of extra hardware.  In this case the USB Host Shield as detailed on the Arduino website:

http://www.arduino.cc/en/Main/ArduinoUSBHostShield

In the list of supported devices is:


USB to serial converters: FTDI, PL-2303, ACM, as well as certain cell phones and GPS receivers. 


The magic word there is ACM - that (or more specifically CDC/ACM) is what the Arduino USB Serial interface is.