It is strange that every good answer here has exactly two downvotes each. Would it be sensible to revert this, or should we not care?
There's nothing moderators can do about this, the information they can access about votes is not fine-grained enough to see if anything strange is going on here.
I would generally not worry too much about small patterns of potentially bad votes, in the grand scheme of things a few votes have almost no effect. But if you stumble upon a larger abusive pattern of votes, just flag one affected post and explain what you observed.
Even though I previously said that moderators can't see or do anything about this specific situation, they can still ask SE to investigate and intervene. I don't think this is necessary in such a small case, but the option exists if someone is trying to abuse the voting system in a large fashion.