If a paper has co-first authors, is the next author after the first authors considered the second author?
In this case, first authorship is being used for some sort of competitive ranking.  As such, if there are two first authors, then in the typical custom of competitive ranking there is no second author: the next author is the third author.  

If this is being defined and used for a specific purpose by a specific institution, however, it is possible that institution has its own interpretation of rankings, which can only be determined by consulting the appropriate people or documentation at that institution.