Dahlquist is configured to put the same video signal out both ports -- the one port which is connected to the monitor which you usually use, and the second port which is connected to the video cable which goes to under the table. So this is just like the usual laptop presentation configuration.
However, it only does this if it thinks there is something on that second port. So if the projector is not getting any signal at all, you have to restart the X server after the external projector (or monitor) is attached. Either log out, or hold down ctrl and alt and press backspace. Either of these operations restarts the X server, and thus dahlquist will re-observe the active monitors at that point.
If the projector shows that the signal is out of range, that would be because dahlquist is set to a resolution of 1280x1024 (the default), but you are apparently using a projector which cannot do more than 1024x768. So you'll need to change the screen resolution, as follows.
Or if the image on the projector doesn't encompass all of the image you see on the computer monitor, you will also need to change the screen resolution.
To change the screen resolution, you need to go to a terminal window (go to the Applications menu, the Accessories sub-menu, then select Terminal) and type "nvidia-settings".
Select "X Server Display Configuration" (second item in the list).
You may be able to click on the two monitors independently. You'll want both their resolutions to be set to the same thing. Press the 'Apply' button. If you can't see what's going on any more, press the ESC key on the keyboard to go back to the previous settings. Otherwise, you will have a choice of keeping the new settings or going back to the previous settings, and you probably want to choose to keep the new settings.
To go back to the full resolution, go to the Screen Resolution preferences program again and set the screen resolution to 1280x1024.
If you mess up your resolution settings and want to reset everything, remove the files ".gnome2/monitors.xml" and ".nvidia-settings-rc" (note leading dots) and log out and in again. If you can't even log in to remove these files, simply press ctrl+alt+F1 to get to a text console, where you can log in and remove files, then log out, then press ctrl+alt+F7 to get back to X-Windows. And you can always press ctrl+alt+backspace to get the X server to quit, if things are so messed up that you can't even manage to log out normally. (Ctrl+alt+backspace is also an acceptable way to log out normally in linux X-Windows.)