Sorry, it was a basic setup issue.The "Extend display screen" setting in the control panel's multi-display settings was not properly configured.The problem was resolved by reverting this setting.
Up until a week or two ago, I was able to output to two displays (via HDMI and Display Cable) from my XPS8940.However, now I can only output to one display (either the display at the end of the HDMI or the display at the end of the Display Cable).The OS and BIOS are up-to-date. I have looked at the following, and it seems to me that it is possible to output to two displays at the same time.[URL] If you have any idea what the cause might be, I would be very grateful for your comments.
XPS8940 no longer outputs/displays to 2 displays (via HDMI and Display cable)