yes you can, but it is unsupported by intel.
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PENTIUM ® III PROCESSOR SPECIFICATION UPDATE
?While Intel has done nothing to specifically prevent processors operating at differing frequencies from functioning within a multiprocessor system, there may be uncharacterized errata that exist in such configurations. Intel does not support such configurations. In mixed stepping systems, all processors must operate at identical frequencies (i.e., the highest frequency rating commonly supported by all processors).
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link to the *pdf
my dad is running a 600EB and a 667 on his i840 board. works just fine