Actually, that's not quite true. The 68K has as many provisions for multiprocessing as every other CPU, namely the necessary synchronization primitives TAS and CAS (and some members even CAS2), plus cache-snooping. Thus, as far as the 68K series is concerned, there is no show-stopper.
There is a practical show-stopper against SMP, and that's AmigaOs, and the Amiga hardware, which - natively - does not support CAS and TAS. As long as a program runs in local memory (not on Zorro, not on chipmem), and only assymetric multiprocessing is concerned, this is a possibility. Any additional core (wether virtual or real) is then bound to run helper tasks beyond the exec scheduler.
That's possible, though that is as much multiprocessing as the Copper is multiprocessing as well. It's just a more complex second processor that speaks, by chance, 68K machine code and not copper machine code.
None of that will give you multicore multitasking in AmigaOs. The Os is a toy-system with the wrong Os primitives.
I think kalamatee was talking about the AROS 68K platform not 68K in general.
