There's no built-in language construct for parallel processing, which means it would be up to the programmer to build task offloading into their sketches. How that should be done depends entirely on the application.
E.g., an application might have one IC doing sensor management and another board doing computational stuff. They might communicate via i2c.
OTOH, for something akin to a general-purpose computer, you would want them to be able to shuffle processes back and forth as needed. Probably with a parallel data bus between them for speed, since i2c might take longer to transfer the relevant data than to just process it locally.
In other words, there's no one-size-fits-all load-sharing scheme, which would be the lion's share of the programming, and a significant part of the PCB design. You save very, very little effort by designing a system generic enough to service either application, since it would have to be customized to the point of being nearly a complete redesign to be useful. On the other hand, a specific implementation would only fit a very small number of people trying to use it in exactly the prescribed way.
In short, those who are interested... well, may or may not have the skill set, but if they were capable of doing anything meaningful with it, they would likely have the skills to design it themselves. Arduino dev boards aren't difficult to design afterall...
Sorry if this comes across harsh. I don't mean it to be, but there's a certain reality to this problem. As is the case with many great ideas, if it were easy or desirable, it would probably be done already. 