On further reflection, I suppose if I had a project in mind that was just a little more than what an ATmega could handle, rather than rushing into a whole new architecture, I probably would consider multiple processors, but not to perform parallel processing on the same data stream. I would, as would many others here I'm sure, try to find one or more parts of the process that could be "farmed out" to perhaps an ATtiny, for example dealing with and partially processing the results from a particular sensor or communications channel, relieving the main controller of some burden. I2C, as already mentioned, would be an obvious choice for comms between the ATmega and ATtiny.