I am running Debian Sid and i was able to get windows 7 and xp to run in seamless in dual monitor mode, with 3D acceleration, you need to install your guest additions in safe mode.
the steps i took were:
install os with dual screens enabled already, if your o/s is already installed i am unsure if you will get support, though i would recommend following the below to do so.
after install hit F8 while booting up windows
select run in safemode with networking (networking isnt really needed, but thats the step i took)
then install guest additions 32bit (dual screens works in 64 but 3D does not)
reboot and you should have dual screens available with 3D support on One monitor (:1)
i couldn't get it to work on both. I did Not have vrdp enabled to do this and it worked just fine.