Contrary to popular belief, an Eyemancer does not need to master multiple disciplines to attain this rank. This is one of the Eyemancers' secrets.
We know that Isaac has mastered Thinkamancy and Lookamancy and is an Adept of Foolamancy, but Roger Victor Clarence is also referred to as a Headmaster, and is only a Master of Thinkamancy, an Adept of Foolamancy, and a Novice of Lookamancy. This would seem to imply that "Headmaster" is a title that refers to mastery of one Eyemancy discipline, combined with some expertise in the use of the other two disciplines. Perhaps, if Novice is "worth" one level, Adept is "worth" two levels and Master is "worth" three levels, then a Master level Thinkamancer needs at least three levels of expertise (the "equivalent" of being a Master) in Foolamancy and/or Lookamancy to be considered a Headmaster.