I could never get this working. Have you got an example of your compiler command.
In case that's still helpful after a couple of years...:
dmd path1/to/source.d path2/to/source.d -o- -D -Ddpath/to/doc -op
generates `path/to/doc/path1/to/source.html` and `path/to/doc/path2/to/source.html`
