What is a circus? It is a company performers/artists of various skillsets that is choreographed, and set usually in a ring or rings.
A headcircus, unlike a headcase, is someone, usually an artist that has multiple skillsets and can easily put them on display for your entertainment.
You may be tempted to think of the circus aspect of this word as a term defining chaos or loss of cognition; when in fact the opposite is true. A circus is an extremely well-choreographed show, with extremely talented individuals performing. The chaos is actually well-controlled.
Another usage of this term is to describe someone afflicted with one or more variations of the affliction ADD (attention deficit disorder) who can use these abilities as a positive instead of a negative (turning a curse into a blessing).
He can sing, dance, draw, paint and play the guitar? That guy's a total headcircus man.