|Could mean that the video card is set to a resolution that this monitor does not support.|
Or it could mean that the video card does not support dual monitors. Just because a video card has two connections, it doesn't necessarily mean it supports dual monitors. I could be one or the other but not both.
Where does the splitter cable come in to this. If you are trying to feed two monitors for the same socket via a splitter that is never going to work.
Like most things to do with computers, knowing the make and model of you video card and the make and model of your monitor and I definitive answer may be available instead of just guessing.