On the new SE sites then you get a little "joining up" bonus if you've sufficient rep on the main SO sites (about 100, I think), but in general I would say that each site is for a slightly different community and just because you've been successful in one doesn't give you the right to have a say over one of the others.
]]>This is not as one-sided as it sounds, though. Questions and answers on MO exceed the math.SE standards, and it would make sense to count them there. The converse is not true, however.
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