As far as I can tell the method you used should work. At least it works for me. However, for me, there was a slight twist. I don't know if it is relevant to your problem, but anyway: on my system (debian etch), I can be logged in as normal in one session and then I can do a ctrl+alt+f1 and get to a text console and log in as some other user and then do e.g.
startx /usr/bin/fluxbox -- :1
and it starts up a new X windows session. Oddly, it is available at ctrl+alt+f2, and I would have thought that it should have been at ctrl+alt+f8. I also would have thought that ctrl+alt+f3 (or f4, f5, f6) would be text consoles, but in fact they are blank. Not sure why that is.