Here's a thread that might help.
http://www.crossdressers.com/forums/...=Abbreviations
The definition in that thread is as follows:
BG / DG Bi gender / Dual gender:
A person who possesses and expresses a distinctly masculine persona and a distinctly feminine persona. They are comfortable in and enjoy presenting in both gender roles.
So, pretty much a bi-gendered person is someone who has two distinct genders within themselves. They sometimes present themselves as a man and sometimes present themselves as a woman.
