Currently impossible.
It is currently impossible to have what you are talking about. The closest we have come is the graphics card, which has a processor running at a different frequency as the processor(s). Also there are cards which have processors that by-pass the primary processor (processor upgrades). The problem with what you are suggesting is that timing is everything in computers, especially the processors. I don't know the exact specs on why, but essentially, the processors wouldn't be able to communicate correctly, and therefore instructions would get all jumbled. Unless they come up with an innovative way to design the hardware and created software that could time instructions correctly, I don't see this happening. I don't really see the advantage to this, except in dedicated tasks like graphics, sound, etc. Maybe if they start making processors designed just for certain functions like a SIMD dedicated processor, a floating point dedicated processor, etc, all feeding off of a primary processor, then we may see some reasoning behind it. But honestly, I don't see anything like that happening in the consumer field, and as it is, processors work most efficiently by themselves, so I don't see a push for this sort of technology.
