I am already running a simpler (albiet slightly more expensive) solution: running a multicore processor. Essentially it is more than one processor on a single chip. In the future this will be the dominant high-performance computer technology-- multiple processor cores at a lower speed use less power, emit less heat, and may cost less than one fast single-core processor.

2 motherboards is not entirely practical, but there are coprocessor addon cards for older computers. This is not common for contemporary computers.