Was there ever any real shared-memory symmetric multiprocessor 68k machines? I know there were several computers that incorporated multiple 68k processors dedicated to separate tasks (or running separate OS kernels in parallel, ala the "Radius Rocket" or multiple-cards-in-one-chassis VME bus boards with dedicated RAM), but I don't know of any shared-image 68k machines offhand. None of the NetBSD 68k port pages mention SMP.

There were a few shared-memory (but not fully symmetric) 80386 machines, like the Compaq SystemPro. It was supported by special versions of SCO Unix and Windows NT. (I think Linux had experimental support for it as well at some point.) That's the slowest "mainstream" multiprocessor "Personal Computer" I can think of. (I'm not counting multi-CPU VAX machines or IBM System/360s from the 1960s and 70's, obviously.)