If you are using virtualbox, probably you get the following error:
failed to initialize KVM: Device or resource busy
No accelerator found!
Just turn off virtualbox and the error should disappear.
The above instructions have been used successfully also on Slackware64 13.37.
First start a new X server (:1) nested inside the current X server (:0)
$ Xnest :1 &
then start a desktop environment in the new X server (:1)
$ fluxbox -display :1 &
$ blackbox -display :1 &
$ wmaker -display :1 &