The word bigender, which (legend has it) emerged from the transgendered community, is broadly accepted as the label for a person who manifests behaviors of either females or males at various times.
The word itself is composed of two sub-elements:
"bi", meaning twice, or two, and;
"gender", the term commonly encompassing one of the two reproductive roles, either male or female.
Since there are good labels for people who physically overlap anatomically (hermaphrodite, or androgyne), bigender's special job is to describe behavior, not anatomy. I submit, though, that a hermaphrodite or androgyne can also be bigendered - so could someone whose physical sexual characteristics are ambiguous (this happens a lot more than you might suspect… between 4 to 8 in every 1000 births - 0.4% to 0.8%)
So far bigender is not a term adopted and used in the medical/psychiatric world for diagnostic purposes. This blog entry is intended to shore up the existing definitions used in discussing gender identity and gender expression.
I’ll venture a little farther and suggest that one can be bi-sexual and be bigendered, but that bi-sexuals are not bigendered by definition.