[anyfoo]

Speaking of which: http://www.os2museum.com/wp/the-nearly-ultimate-386-board/

The article describes a motherboard with both a soldered on 386 CPU, and a separate socket for another one. This was not a multiprocessor board, to make use of the socket you were supposed to disable the onboard CPU using a jumper.

But if you plugged in a similar enough CPU without setting the jumper, it appears that both CPUs ran at the same time. And since they had the same clock and they should have no difference in behavior, the system, at least as tested, ran fine.