The dual 328 board came out of this threadhttp://arduino.cc/forum/index.php/topic,115711.0.html
It is the equivalent of 2 arduinos less the USB/Serial interface chip on one board with RS232 level signals available on a header to connect either/both uC to a DB9 type connector.
Dual 6-pin FTDI header allows either uC to connect to USB with an FTDI cable, FTDI Basic, or connection to FTDI Breakout board or CP2102 module.
The original idea was to provide dual processing capability with SPI, I2S, or UART connectivity between the cards with user added jumpers.
I tried to make it with a single crystal between the uCs, but it got to be too much hassle (or perhaps just my inability to use avrdude at the command level) - so instead each uC can be bootloaded via their own ICSP header as an Uno (optiboot) and no fooling around getting fuses set up is needed.
One could always go that route with a jumper from XTAL2 on the crystal-equipped to the other processor's XTAL1 input. See '328 datasheet 9.4 and 9.8. Some fuse setting changes are needed.