I'm afraid this is not possible, as far as I'm aware the login GINA screens are only written for a single display source, thus preventing you doing what you are intending to do.
There may be some obscure tool out there to do it but I'm certainally not aware of one and have not come across one in over 6 months of looking, albeit on and off. ( I was trying to do the same on some of our Designers PC's)
on the plus side though there are many open source GINA's out there. perhaps there is one that suits our needs. i was reading an interesting article about them earlier: http://www.windowsitpro.com/article/security/pgina-open-source-gina-replacement.aspx