Are you trying to save on power consumption? My gut feeling tells me that you'd be better off not using a core at all than using both at full CPU with a lower clock speed. I could of course be dead wrong, I have seen platforms that consume much less power at lower clock speeds than while halting, but x86 platforms these days should be saving a lot while CPU isn't use.
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If this isn't your goal then I have a hard time imagining a multicore x86 CPU that can't already run VBA well enough. Are you thinking about embedded non-x86 platforms or something?