Perhaps it is not an issue in sites like MathOverflow because it is really shameful for a prestigious professor to get involve in such childish games but in Math.Stackexchange it could happen.
Here there are many users who greedily seek for higher reputation thresholds. According to the site policies establishing satellite accounts and voting between them is not allowed and the user will be punished if this behavior is spotted.
But what if this voting irregularities happen between different users as a kind of agreement? A user A upvotes almost every post of another user B. In response the user B votes for almost every post of the user A. Both will get higher reputations and this is not "illegal" because they are different users.
What is the site's policy in such cases?