I've been looking for a way to do the same thing.
While still quite clumsy, this is the simplest method I have found so far:
You will need to install Xephyr (which lets you start an X server in a window) and a light window manager like xfwm4:
sudo apt-get install Xephyr xfwm4
Then start up Xephyr with two screens:
Xephyr +xinerama -screen 800x600 -origin 800x0 -screen 800x600 :1 &
Then start a window manager on the new server:
DISPLAY=:1 xfwm4 &
Then start impress:
DISPLAY=:1 libreoffice --impress &
I wish it weren't this complicated. Having an option in Impress to view the console on a single monitor setup would be very useful when practicing presentations.