> Some games uses multi CPUs... I suppose it would be possible to distribute each
> on separate threads so that each core can process one. Maybe this is already
> done, I'm not familiar with MAME's sourcecode in this area... anybody knows?

There's like one monster thread each month on mame.net/msg about this "issue" and the bottom line is : not worth the hassle. Multi-CPU games run full speed on today's single-cpu PCs and there's no speed gain splitting CPU and graphics on, say, sfrush - same framerate with or without rendering. Splitting one emulated CPU on 2 physical ones would probably generate more overhead than speed and no one wants to undertake this kind of experiment since it would be a lot of work for very uncertain results and an obfuscated source code.
That said, PCSX2 seems to have a lot of multithreading going on and people who are interested in the subject should look at that project instead of MAME - it makes more sense in every way.