Whoops, I just saw your updated post. I agree, keeping it simple is best. Having each DSP be independent and not crossing data lines keeps a lot of complexity out of the design.
As far as addressing multiple devices in the system is concerned, it's quite simple. Every I2C message starts with a chip address byte. All you need to do is change the chip address byte every time you're accessing a different slave device on the I2C bus. SigmaStudio can help you generate your microcontroller code, or at least a big part of it (see here for more details: How do I create the microcontroller code to interface to my SigmaDSP?). You'll notice that in the exported files from SigmaStudio, there is a definition for the I2C address. You just need to change the address to match what's in your actual hardware.
You can also take care of this in SigmaStudio by setting the proper addresses on the USBi. Check out the "Configuring the USBi to communicate with multiple ICs" section on page 4 of the USBi application note, here: http://www.analog.com/static/imported-files/application_notes/AN-1006.pdf