Theoretically yes, although this would be an advanced hack. If you try this, be sure to shut off the power to the GoPort on both boards so that they're not trying to power each other. You'll also need to implement SPI Slave for the second board.
So I should be able to plug to NGo main boards together with a GoBus cable and get 21 mbits between them?
To set your SPI clock to 21mbps, you can use a SPI clock divider of "HSE/8".