User-agent: Mozilla Thunderbird 1.0.7-1.4.1 (X11/20050929)
The initial perspective is the one that your workbench window has when
open ... and it can only have one.
If you really want to open a couple of perspectives, isn't there some
kind of workbenchAdvisor post-open method? You could track down the
workbench window or active page and open some more perspectives, if you
really wanted to.