If you have read the Rocheworld series by Robert L. Forward, you know what this is.
A binary planet is a planet that is actually a double planet which orbits about a barycenter.
Attached are two images exported from Celestia.
TMBinary.jpg - Image showing Tyinbra eclipsing her twin,
MTBinary.jpg - Image showing Makadran eclipsing his twin.
I have been looking through the program and the PDF doc, but I cannot find how to add a binary planet.
Thanks for any help.